Pixar has never made alot of mistakes in their movies. and the only other one I can think of at the moment is RC losing his remote in Toy Story on the road but in TS2 it’s back.
But I always thought this one of A Bug’s Life was funny:
Flik is about to embark on his journey, one of Dot’s trouble-making friends says that his dad thinks Flik is likely to get eaten. While the other kid says that his dad thinks Flik will die.
Yet when Flik glides across the canyon and hits into the boulder, the kid who says “your dad’s right, he’s gonna die”, was in fact the kid with the father who said that!
I know it’s not a big deal, just always wondered if anyone else ever heard it. 
