Revealing mistake: When Homer placed the rocket on his fence and lit it his friend asked "Should we get behind somethin'?" That's when the rocket explodes. When you see them fly back you don't need to slow motion it because you can clearly see the wires attached to their backs.
Revealing mistake: At the party where Inspector Gadget lights a cigar for Scolex, he uses his thumb which flips open to reveal a flame. When they cut to the close-up of Scolex getting the cigar lit, you can see that there is a hand holding up a traditional lighter, and not Inspector Gadget's thumb any more.
Revealing mistake: When Whiplash is in the cave and Dudley comes and checks it out, the cave falls in. You can see a black curtain fall down while the screen goes black.
Revealing mistake: When Sly breaks the board you can tell it was already broken.
Revealing mistake: As Lawrence III walks up the stairs to his console, he clips through them in the process. (00:01:43)
Revealing mistake: At the end of the film, the screw is tapping on the window to get out and Hogarth moves to put the box down. The box completely vanishes from the shot for a few frames towards the end of the shot.
Revealing mistake: When the toys are watching Woody being stolen the games machine on top of the TV doesn't have its controllers plugged in, but Rex was playing earlier with a wire coming out of his controller, which clearly wasn't actually connected to anything.