When a show has been around as long as Supernatural, it is easy to make mistakes, whether they are plot holes, continuity errors, or head-scratching inconsistencies.

With 13 seasons under its belt, the battles between demon-hunting brothers Sam and Dean and both the forces of Heaven and Hell in Supernatural have resulted in monster ratings for the series.

Fans have supported it through thick and thin to ensure that the series has lasted longer than just about any fantasy television series in history. However, with the show about to hit 300 episodes this season, there have been plenty of mistakes -- many of which the fans of the show have noticed but ignored for the most part.

While there are plenty of monsters of the week episodes, there is also the overlaying mythos of Heaven, Hell, and the fate of the brothers.

With so many angels and demons coming and going and so much mythology thrown in, it is easy to see how the writers could get lost in its own deep story and make mistakes along the way.

While fans have tried to explain away some of the mistakes, others are just glaring plot holes that make dedicated long-time fans scratch their heads and wonder why the writers don't pay as close attention to the show as its legion of fans.

Here is a look at the 20 Mistakes Fans Completely Missed In Supernatural.

CASTIEL CALLING HIMSELF GOD

In the Supernatural season 6 finale, Castiel appoints himself the new God. He also tells everyone -- friends and enemies -- that they are to respect him as such or perish.

It was a moment when one of Dean and Sam's allies went a little over the line and became a very dangerous being indeed.

However, there is a huge problem with Castiel deeming himself The God. God was still alive and living on Earth as Chuck.

Sure, there are other gods on the series, but Castiel informed everyone that he was The God, which is not possible since The God was still alive and well.

Over time, Castiel decided that he didn't want the responsibility anymore and went back to just being an angel.

However, becoming God made no sense on the show.

SAM'S INCONSISTENT AGE

There was an error very early on in the history of Supernatural and the show just glazed over it and kept going without missing a beat.

In the very first episode, Dean heads out to find Sam at his college and tell him that their dad has disappeared and he needs help finding him.

This was when fans learned that Sam was preparing to start law school. The only way to get into law school is by completing a four-year basic college degree first.

In the pilot episode, Dean mentions that Sam has been away at college for two years.

Creator Eric Kripke said in 2006 that they made a mistake in the script and Sam was supposed to be 20, but they just changed it to 22 and moved on from there.

REAPERS ARE ANGELS, BUT ONE WAS POSSESSED BY A DEMON

When it comes to inconsistencies within the world of Supernatural, look no further than the Reapers.

These creatures show up when someone is perishing and then take them into the afterlife. The first Reaper appeared in the very first season of the series.

Overall, they were neutral in the entire scheme of things and took no sides, simply doing their jobs.

However, in season 8, the show introduced rogue Reapers, which could smuggle people in and out of Heaven and Hell for a price. There were also rogues that served Crowley.

Then, in season 9, it turns out that Reapers are angels -- even though the first Reaper they met in season 1 was possessed by a demon, which is something that is impossible for an angel.

It seems that the show has no idea what a Reaper is or isn't.

GRANDPA WINCHESTER LEAVING HIS FAMILY

Possibly, the most glaring error that many dedicated Supernatural fans found involves the character of Henry Winchester -- the father of John Winchester and the grandfather of Sam and Dean.

In the first few seasons of Supernatural, the show made it clear that Henry had abandoned his family, and in season 8, it showed that the reason why he left his family was that he was a time traveler and passed away in the future helping his grandsons.

There is a huge problem here.

In season 4, Dean travels back in time and sees his dad, who at the time is in his early 20s. Someone then asks John to say hi to his dad, and John says he will.

This is impossible if Henry left his family when John was a young child, as season 8 makes clear.

THE INCONSISTENT VISIT TO THE GRAND CANYON

Not all of the mistakes in Supernatural are big and involve inconsistencies concerning demons. The Grand Canyon error is one example of a minor mistake that never should've happened.

In the second season episode "Croatoan", Dean says that he and Sam had never been to the Grand Canyon. However, in "The Great Escapist" in season 8, Sam said that they went when he was four-years-old.

When it comes to the number of landmarks in the United States that Supernatural could namedrop, the idea that the show uses one and then contradicts itself a few seasons later by using the same one makes no sense.

Writer Ben Edlund went to Twitter to admit to the mistake.

THEY SHOULD ALREADY KNOW WHAT RUGARUS ARE

Dean and Sam have an entire journal of their own and their father's concerning monsters and how to defeat them.

It has come in handy when they needed to learn how to beat a monster that they have never encountered in their lives.

This happened back in the early years of the series when Dean and Sam would meet monsters at the same time that the audience saw them for the first time.

In the season four episode "Metamorphosis", the boys met a Rugaru, a being who is human until they turn 30. After this, they develop a taste for human flesh.

They had never heard of a Rugaru before and have to learn how to deal with it.

Jump forward to a season nine episode titled "Bad Boys", and a young Dean in a flashback is saying that his dad was out hunting a Rugaru.

SAM AND DEAN BECOMING 2D

One of the fan-favorite episodes of season 13 had Dean and Sam getting sucked into a television. There, they found themselves teaming up with Scooby Doo and Mystery Inc. gang to solve a crime.

See, Dean was a massive fan of Scooby Doo and this was very exciting for both him and any fan of Supernatural who grew up watching the classic cartoon series.

However, there was a problem with the episode.

In "Scoobynatural", Dean and Sam were sucked into the 2D cartoon -- Dean actually said in the diner that they were converted into 2D.

There's one thing that doesn't quite add up: if 3D characters are sucked into a 2D world, as they said, how can they see anything? Of course, this is Scooby Doo, so most fans just ignored this error.

THE DIFFERENT KINDS OF GHOSTS

Supernatural seems to change the rules as the show goes on when it comes to ghosts. As a matter of fact, the series tries to show early on that there are differences between ghosts and poltergeists -- but then they changed those rules as the seasons moved on.

From the start of the first season, poltergeists are introduced. They are different from ghosts since they are made up of negative energy.

However, in the season 5 episode "Swap Meat", the brothers hunt a poltergeist that it is the ghost of a deceased witch. However, she is still considered a poltergeist.

Also, in the season 7 episode "The Girl with the Dungeons and Dragons Tattoo", Bobby's ghost gets agitated and Dean says that all ghosts go bad.

However, this is in conflict with previous episodes where ghosts helped the brothers on occasion.

HOW TO TAKE OUT A ZOMBIE

Ever since George Romero created the modern day zombie in the seminal horror movie Night of the Living Dead, the rules were simple. In it, the only way to take out a zombie was with a headshot.

However, with Supernatural, the show has always tried to change up the basic mythology of monsters to make them unique and different for their fans.

Because of this, there are some cases where the show seems to change its mind about monsters as time moves on.

The first zombie on Supernatural appeared in the season 2 episode "Children Shouldn't Play with Dead Things". In this episode, Dean and Sam reveal that the way to eliminate zombies is to lure them back to their grave and stake them in their coffin.

However, in season 5's "Dead Men Don't Wear Plaid", they are able to take a zombie out with the old headshot trick.

So, which is it?

DEAN TAKING OUT MONSTERS IN PURGATORY

Purgatory makes almost no sense on Supernatural. For one thing, why is there a backdoor to Purgatory? If this is where the sinners and monsters go to stew, why would there be backdoor at all?

While this is puzzling, it is more of an unexplained plot device and less a mistake. However, there is one plot hole that was never answered: where do things go when they pass away in Purgatory?

See, Purgatory is where sinners and monsters go when they expire on Earth and they don't get into Heaven or Hell. It is the in-between, which means that they are no longer alive when they end up there.

In the season 8 episode "Blood Brother", Dean offs monsters in Purgatory but they never explain how this is possible and where the monsters go next.

SAM AND DEAN'S UNLIMITED MONEY SUPPLY

While this is something that the show has explained, it still makes no sense. Dean and Sam, from the start, are able to drive across the country, stay in motels, buy food and clothes, and survive without having a real job.

Where do they get their money?

As the show reveals from the start, the boys get credit cards under false pretences and then use them without care since they consider credit card companies to be immoral anyway. Also, Dean is a pool hustler.

With that said, where do the credit card companies send the cards?

Sam and Dean never stay in one place long enough to have a card mailed to them and they can't use one post office box or address or they would get caught.

Plus, when does Dean have time to hustle pool when they are constantly battling demons? It seems like an easy explanation, but it's one that really doesn't hold water.

LEAVING ADAM IN HELL

Supernatural fans have gotten used to plot holes. They are so used to them, in fact, that one plot hole has become a meme in and of itself.

In the season four episode "Jump the Shark", it turns out Sam and Dean had a half-brother named Adam Milligan who perished at the hands of ghouls back in 1990. I

n the season 5 episode "Point of No Return", Adam is resurrected and once again is a part of his brother's lives.

In the episode "Swan Song", Adam is trapped in Hell in the Cage with Sam, Michael, and Lucifer. Dean can only save one and chooses Sam, leaving Adam in the Cage.

It has been shown that there is a way to get beings out of the Cage, but after seven seasons, Adam is still there.

The season 10 episode "Fan Fiction" even pokes fun at them for forgetting about him.

ANGELS ENJOY EATING... EVEN THOUGH THE FOOD TASTES LIKE MOLECULES

There are several things about the angels that make no sense. However, one of the things that the show seems to waffle on is their desire to eat food.

See, in season 9 episode "First Born", Castiel is complaining about becoming an angel again. In the episode, he said that one of the things that he will miss the most is enjoying food because angels can't taste food as it just tastes like molecules to them.

However, this completely goes against the season 5 episode "My Bloody Valentine", where Castiel is seen eating hundreds of hamburgers due to the influence of Famine. When asked about the food, he says that he thinks they taste delicious.

Add in the fact that both Balthazar and Gabriel are shown enjoying eating food, and Castiel saying angels can't taste food like humans do makes no sense.

WHERE DID THE COLT GO?

When it comes to the Colt, it is less of a mistake and more of a question of where it went and why Dean and Sam were not trying to get it back after they gave it up.

This was a perfect tool that they used that could beat almost any creature they encountered. This meant that it was possibly too powerful for the show and thus is why it needed to go away.

However, this does not explain why they didn't care so much about recovering it, as it would have made their hunting expeditions even easier.

The weapon is introduced in season 1 and was very useful. After the season 6 episode "Frontierland", it disappears until season 12, and no matter what they faced, Dean never felt the need to retrieve it.

THE DISAPPEARING ANTI-CHRIST

Jesse Turner appeared in the season 5 episode "I Believe the Children Are Our Future". He looked like a cute little kid, but he was actually the son of a demon and human.

Not only that, but little Jesse is also the Anti-Christ -- yes, according to Castiel, Jesse could eliminate all angels in Heaven with just one single word.

Castiel rightly wanted to destroy the child before he could, in turn, destroy Heaven and Earth. However, Dean and Sam had other ideas and allowed Jesse to live, convincing him that he could use his powers for good.

The problem is that this is the Anti-Christ and the harbinger of the end, yet he has never been seen again since season 5.

Plus, with Jack Kline as a possible Anti-Christ, could there be more than one?

AZAZEL TELLING DEAN TO LEAVE SAM

The first Big Bad in the world of Supernatural was Azazel -- the yellow-eyed demon. He was the one who set the entire show in motion after taking out Mary Winchester and Sam's then girlfriend to force the brothers into action.

What was Azazel's plan? His goal was simply to use Sam in order to free Lilith. He then had someone working for him in season 2 named Jake Talley try to eliminate Sam in "All Hell Breaks Loose".

Azazel also tried to convince Dean not to bring Sam back from the other side.

There is a problem with this, though: Azazel worked for Lucifer. As Supernatural fans know, Sam and Dean were destined to be the vessels for Lucifer and Michael for their final battle.

Why would Azazel want Sam to go away if Sam was needed for Lucifer's final plans?

NEVER USING CHRISTO

"Christo" was an annoying addition to the Supernatural mythos. Christos is the Latin term for "by Christ" and by saying this one word, a demon flinches.

Just one flinch gives experienced hunters like Dean and Sam the one second that they need to pull the trigger and eliminate the evil without the need for a prolonged battle.

In the first season episode "Phantom Traveler", it is Sam who mentions the word "Christo" and explains what it means and how it can help them when battling a demon or a possessed person.

With that said, they never used the word again in the next 12 seasons, despite having numerous chances for it to help them win a fight with a demon.

ALL OF THE INNOCENT PEOPLE SAM AND DEAN HAVE TAKEN DOWN

With all the laws that Sam and Dean have broken -- and the fact that they are on the FBI's most-wanted list -- there is no reason why they shouldn't be rotting away in jail right now.

With that said, most fans can just dismiss this by stating that the brothers are doing the right thing at the end of the day.

However, in recent years, there is one thing that Sam and Dean are doing that is unforgivable.

Head back to early seasons of Supernatural, and Sam and Dean would save people who were possessed by demons through exorcisms and so forth.

However, as the years have gone by, the brothers no longer seem to care about the innocents that the demons possess and just take out the demons by eliminating the hosts as well.

This means that the brothers are taking out innocent people for the greater good. This goes against their motto of "saving lives."

THE INCONSISTENT PREPARATION TO BECOME VESSELS

The mythos of Supernatural eventually revealed that Sam and Dean Winchester were the chosen vessels for Lucifer and Michael -- and this was set in stone since the days of Cain and Able.

The idea of brothers being on the opposite ends of the battle between Heaven and Hell was very intriguing. However, the fact that they were vessels brought up a huge inconsistencies in the series.

Sam had to drink massive amounts of demon blood in order for his human body to be strong enough to host something as strong as Lucifer. Otherwise, he would explode.

In season 13, Dean just willingly allows Michael to take over him as his vessel with no preparation whatsoever.

Why is there a difference when both entities are equally powerful?

THE ANGEL CONTRADITIONS

One would think that Supernatural has a clear idea of what angels are and what they can do. Castiel is one of the main characters on the show and he is an angel, so this makes sense.

However, there is no real strict guideline to what it means to be an angel. Angels have banished other angels in the series without it affecting them, but on other occasions, both angels are banished.

At one point, burnt wings appeared next to a fallen angel, and then in season 9, the wings burned off completely.

When angels first returned, the show revealed that they had not been on Earth for thousands of years. However, in "Lily Sunder Has Some Regrets", angels are said to have been on Earth 100 years ago.

Add in changing powers and weaknesses from season to season, and angels become a confusing mess.

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Can you think of any other mistakes in Supernatural that only the most dedicated fans noticed? Sound off in the comments!