Fans may never know why Wolverine's metal claws and bone claws look so different, but we know this much: audiences weren't supposed to see the screws in them in the first movie.
Cyclops may have an awesome power, but he needs to manually fire his optic blasts by pressing his visor - except in the movie's opening, when it fires a controlled blast all by itself.
It turns out Magneto can't just control metal - he can control time itself, going by the reversed footage sold out by the exhaust flowing back into a police cruiser's tailpipe.
Most comic fans have a hard time remembering Iceman's real name - including his co-star, Anna Paquin. When the two meet on a school bench, the word "Bobby" is heard, even though she's clearing saying "Shawn," as in actor Shawn Ashmore.
Watching Wolverine take a bullet to the head, and then watching his body push it out is unforgettable. Until you realize his skull is made of the world's toughest metal, meaning the bullet wouldn't have had anywhere near enough flesh to lodge in, and should have just ricocheted.
When Mystique tries to romance Wolverine pretending to be Jean Grey, it's her scars that give her away - scars made by his claws in the first movie. But the scars are now lined up vertically along her stomach - not horizontally, like they clearly were in the first movie.
Nightcrawler's teleporting helps him rescue kidnapped children, but instead of filming the scene twice, they did it once - which means even before he BAMFs into the cell, you can spot his silhouette in the shadows.
Wolverine might heal, but apparently his clothes do, too. Judging by the bloody holes torn in his undershirt by a pair of bone horns (1:06:00) - which completely disappear later in the scene.
During the final battle on Alcatraz Island, overhead images show the Golden Gate bridging easily reaching from the shore. That's because the island in the image is actually way bigger, and closer than in real life - Google it yourself.
Mutants have powers, but they can't all read. Not Pyro anyway, who gets Multiple Man's cell number wrong, despite the fact that he's standing right in front of it.
X-Men: First Class
When Hank McCoy is teaching Banshee to use his scream to fly, he gives an easy instruction. Oh, make the sound waves travel faster than sound, that totally makes sense.
Add another superpower to Magneto's list, since he's able to not just throw an armed guard into the air, but apparently keep him flying into the stratosphere, since he never drops back to Earth again. He will be missed.
X-Men: Days of Future Past
The effects team of Days of Future Past painstakingly recreated Robert F Kennedy Stadium in Washington, down to the groundskeeper painting the baseball diamond. But nobody double checked the year: in 1973 the Senators had already become the Texas Rangers, leaving it only for the NFL's Redskins.
The Paris Peace Accords play a major role in the story, but the famous signing took place in January of 1973 - but you won't find any snow anywhere in the film.
Wolverine uses his bone claws to keep Magneto from reading about those same Peace Accords - but he wasn't missing much: the opening paragraph is taken straight from the agreement's Wikipedia page.