As most of you know, there are many villains who have great and elaborate plans and are evil geniuses, but even the smartest of villains and their plans can have a goof up that often makes said plan fail or lets the heroes get them. There are possible spoilers on this list, read at your own risk.
For how I rank these Goof ups here's what I consider about them:
~How minor the goof up was
~The consequences of the Goof Up
~If a back up plan was present
~If the villain was thinking that his/her plan was fool proof
10. Mass Effect 3 - The Reapers don't turn the Mass Relays off
In the Mass Effect series (though probably not the upcoming game since there's heavy sequel evidence) the Galaxy has become nothing more than an experiment for a billion years ever since one species' mistake created the Reapers. The Reapers are a race of sentient star ships who built the Mass Relays, the Citadel and made the pattern for Mass Effect technology. Every 50,000 years they have a cycle of "harvest" where they kill every species that has managed to achieve space flight and "preserve them" as a new Reaper.
In game its mentioned they do this by sending a signal to a species in the Citadel who hears it as "Its time to harvest, turn off the relays" which shuts down the Mass Relays and basically cuts the galaxy's aid apart. This allows their harvest to be easier, and despite the fact that last species they harvested: the Protheans, made this signal unable to work this doesn't destroy this tactic; the Reapers always leave a single member of their species to watch the galaxy and said member could activate the Citadel's relay system to turn it off, and it could also get the other Reapers to start the invasion.
In Mass Effect 3 however they had began to invade yet they were completely ignoring the Citadel despite shutting off the Relays would have made their job easier and stopped Shepard from getting the resistance that was needed to beat them. The strategy they used most often to cripple any cycle was ignored despite the fact they've had enough Back Up Plans that they have up to a Plan H. And even more, once the Illusive Man warns them near the end of the game and they take the Citadel so its over Earth (which where they had the most forces present to defend it) they still didn't turn them off, they let the army made of the entire galaxy reach Earth instead of turning the relays off and then ambushing them as they tried in vain to reach Earth.
While this goof is a big goof, there's the explanation that this cycle is an anomaly due to Commander Shepard who destroyed Sovereign and became the only thing the Reapers ever feared, its even possible due to Sovereign being unable to do this the other Reapers might all think that the current cycle finally made it so they can't turn them off due to the delay in the harvest. So their goof is most likely the result of the logic they noticed in this situation and therefore not as much a goof as an error in judgment.
9. Pokemon Mystery Dungeon: Explorers of Time/Darkness/Sky - Darkrai underestimates "you"
During the Gen IV Mystery Dungeon games there are two different stories, and both are the result of one Pokemon who just wants destruction on the whole world. In the beginning story you travelled through time and saw the first plan succeed, but you and your partner prevent said future from ever happening ruining that plan, the mastermind behind it soon hatches a second plan up and this time makes you and your partner look like the cause so that you won't think to stop it. However events he didn't prepare for show his identity as Darkrai and the true mastermind behind the entire game itself.
Once you go to fight the final battle and finish him he pulls one final trick and tries swaying you... and your partner agrees to it! Since your partner joined Darkrai tries even more to get you to him, however you realize who your partner really is and break the spell to Darkrai's shock. With that you fight him and his back up, and he tries to cowardly flee to make another plan. Unfortunately earlier Darkrai was stupid enough to reveal your amnesia and how you got it, as he tries to get away Palkia appears and once he pushes Darkrai inside he shatters the portal which causes Darkai's memory to be completely wiped and end his terror.
8. Fairy Tail - Future Rogue's cruelty
During the Grand Magic Games arc there's a mystery lurking around the air with the presence of Zeref's magic being sensed by Jellal, Meredy and Ultear. During the 4th day of the games when Fairy Tail and Sabertooth's dragon slayers are fighting each other Jellal senses this presence and follows it, but he's shocked by what he finds while we don't even know what he's found. On the last day of the games after Natsu, Wendy and Mirajane rescue Lucy and Yukino from prison they encounter said figure and she reveals to be Lucy from 7 years in the future... a bad future where 10,000 dragons ravaged the land.
While trying to escape the guards and the Knights who are responsible for executions another person from the future comes, this time its Rogue from the future. He tells the others he comes from the same future and how the Eclipse Gate has a weapon that can deal with the dragons so he can get Natsu and the others on his side, he also mentions that someone closes the gate before this happens in the past dooming the future and he has to kill them, while the others think there's another way he says killing is the only solution.
The one he has to kill however... is Lucy, right as he shoots an arrow thinking he's sealed the deal, Future Lucy takes the shot to his surprise, Future Lucy states that she didn't close the portal and says her last words as she dies from the wound. With that, none of the Fairy Tail members trust Future Rogue anymore and Natsu makes this clear in one of the best ways any wizard of Fairy Tail can, a punch to Future Rogue's face while claiming no one can take away Lucy's future.