The pride comes before the fall. The Jedi Council knows this all too well. The Jedi Order was practically at the height of its power during the Prequels of the Star Wars films. They sat highly on their perch on Coruscant and in their hubris, even likely forgot that their temple is still built on top of an ancient Sith Shrine.
Needless to say, having this much confidence in their ways have led them to commit mistakes that eventually snowballed into Sheev Palpatine's Galactic Empire. What's worse is that they could have prevented some of these if only they were more unprejudiced or active. Too bad these 10 mistakes have already been committed which isn't too bad as it led to more Star Wars films but still...
Dabbling Into Politics
While he's politically aware, Obi-Wan understands that a Jedi shouldn't interfere in things that aren't their area of expertise. Case in point, politics. As he said himself, he's not brave enough for it. Too bad the Jedi didn't share his fear of courtrooms and podiums because they very much had a hand in the flipping the Senate. Then it turns out the Senate is actually Sheev Palpatine, or more specifically, he controls it.
The Jedi walked right into Palpatine's trap and accidentally helped the Chancellor become more powerful. If only the Council had abstained from meddling into politics, then Palpatine would have had a much harder time painting them as a power-hungry cult and thus gaining support.
Suppressing Anakin's Emotions
To be clear, Anakin Skywalker does have destructive tendencies and would occasionally put himself or others in danger thanks to his impulsive emotions, and this was something that needed to be curbed. However, what didn't need curbing was Anakin mourning his mother's death as well as his anger.
The Jedi do have their code about controlling passion but instead of helping matters, this only bolstered Anakin's baggage and angst. Surely, there were better ways of teaching him to let go of attachment or how to deal with emotional pain other than ignoring them or telling him to get over it.
Letting Darth Maul Control Mandalore
It's safe to say that the Jedi were wholly unprepared for the unveiling of the Sith. They handled it rather poorly and lost a Jedi Master in the process. Then there's also the fact that they let Darth Maul run rampant and even let him take over the whole planet of Mandalore, giving more power to the Sith. Their response? Send Obi-Wan alone to deal with it. Beside, it's just the Sith — why take that ancient threat seriously?
Thing is, Obi-Wan didn't take care of things, so he had to ask for backup. Only then did the Jedi Council send more clones, right after the damage has been done. They lost some valuable allies in the process.
Baselessly Exiling Ashoka
Here's another instance from Clone Wars that perfectly showcased just how arrogant the Jedi Council can be. That, and it also displays how they can't even do anything correctly. Here, they exiled Ahsoka Tano on mere suspicion alone and didn't even confirm whether she was involved in the bombing of the Jedi Temple or not. This despite her being one of the most powerful Jedi in the Clone Wars and someone who could have helped after Order 66 was carried out in the Jedi Temple.
Anakin did prove Ahsoka's innocence and showed the Council just how wrong they were, but Ahsoka already learned a more valuable lesson from the Council the day she was exiled: the Jedi don't look after their own.
Not Making Anakin A Jedi Master
The Jedi Council not only wronged Ahsoka but also her closest friend. Based on their suspicion as well as the hint that Sheev Palpatine is up to no good, they invited Anakin over to the Council as a pseudo-member and his only task was to spy on the Chancellor. Not only was this political meddling once again, but it was also an insult to Anakin.
They could have just given him the task first without wedging him into the Council as a Jedi Intern with zero practical privileges. This was another blow to Anakin's Jedi alignment and allegiance, and even further proves the point that the Jedi are bad at looking after their own members. Besides, they essentially handed Palpatine a new apprentice!
Mace Windu Wants To Kill Palpatine Right Away
For all of Sheev Palpatine's crimes, even something like a quick death wouldn't have sufficed. He had to be tried and punished. Even Anakin thought of this, but Mace Windu wanted to take matters into his own hands and made a swift and reckless judgment on behalf of the Council to kill Palpatine. Oddly enough, they did plan on just arresting Palpatine but Windu changed his mind.
The thing is, if Windu only listened to Anakin, then they could have reversed Palpatine's plan of gaining the sympathy of the Senate. After all, Anakin was there as a witness along with three dead Jedi Masters as evidence. There was really no need for Mace to skip the due process, but he paid for that dearly.
Not Training Luke Or Leia Earlier
It was rather important for the Jedi Order to regain solid footing after getting wrecked by Order 66. That's why they needed every Force-sensitive soul they can get. Even other Jedi such as Ahsoka scrambled to save these said souls or even gather them. That's why it's puzzling to see the Council, or just Obi-Wan, decide to set both Luke and Leia aside.
He had the capacity to take care of at least one of them and shape them into a formidable Jedi without needing to rush training later on. After all, he was in the same place as Luke already, so what was stopping him?
Obi-Wan Lies (From A Certain Point Of View) To Luke
Speaking of Luke, plenty of his injuries (both physical and psychological) could have been prevented if Obi-Wan had been more transparent with him. In fact, Luke could have kept his hand if only Obi-Wan told him that Darth Vader was actually his father.
If Obi-Wan only told everything beforehand, there's a high chance that Luke wouldn't have faced Vader prematurely -- even though their first clash is one of the most iconic movie moments in history. Even Luke himself questioned Obi-Wan's lies, which were supposedly true from a certain point of view. Yeah, right Obi-Wan.
Not Taking The Sith Seriously
In addition to letting Darth Maul take Mandalore and not acting sooner, it seems the Jedi Council brushed off the Sith threat as Qui-Gon probably having had too much gin or simply put, not taking him seriously. This was even after Qui-Gon died to a Sith Lord.
One would expect the Jedi to be more vigilant after The Phantom Menace and start investigating any potential Sith leads, but they played politics and war instead. They also ignore much of the signs revealed to them in the Clone Wars. The worst offender is Yoda, who actually saw Anakin becoming a Sith in a vision.
Going Into Exile
Last but not least, one of the biggest mistakes of the Jedi Council (or what was left of them after Order 66) was going into exile. Looking at you, Yoda. Frankly, staying in some godforsaken swamp planet while the survivors of the Order 66 picked up the pieces and risked their lives to rescue younglings and Force-sensitive individuals reeks of selfishness and self-righteousness.
Even Obi-Wan is guilty of this as he elected to grow old and do nothing in Tatooine, instead watching over some farm boy who he knew was a Jedi Master in-the-making. He could have at least dedicated those wasted decades to helping some Jedi stragglers or training Luke instead of waiting for him to get attacked by Sand People so he can make a weird entrance.