Revealing mistake: Near the end, right before Nita goes after Kenai, she takes off her dress. If you look closely, when she takes it off, her necklace and earrings come off with the dress as if they were a part of the dress. Also, her hair is tied back when she's wearing the dress. But when she takes it off, her hair is loose and flowing.
Revealing mistake: Just after Boog breaks the dam and the deer herd is bounding across the dry riverbed, at the moment they realize they are going to be swept downstream by the wall of water, their legs disappear - just left out of the animation cell. Very obvious; no slow-mo needed to see.
Revealing mistake: In the scene where Luigi offers his service to Van and Minny, Guido is juggling wheels. But when Guido finishes, you can see over his right lift that his one of his wheels is not rendered correctly, as it seems to overstep his bodywork.
Revealing mistake: Right after McQueen asks what the Ghostlight is, it cuts back to the other cars. Luigi is right next to the firetruck and some gas pumps. The firetruck's door is very reflective, and the gas pumps are visible in it. But Luigi is right by the pump, and he is not reflected in the firetruck's door. (00:02:10)
Revealing mistake: When Stella the skunk yells at the cat, she pokes him on the face. However, you will notice that the hand looks a lot like a human hand, not Stella's.
Revealing mistake: Lugnut is driving to place and stops to look around to see where the race betting is. When he stops, it is apparent that there was no animation used to show him stopping, but really, the animation was just paused in attempt to give the illusion that he stopped. (00:17:35)