Deliberate mistake: When Boo laughs repeatedly while playing hide-n-seek with Sully in the men's room, nothing happens to the electricity - no surges as with all other times she laughs. [In the audio commentary by the directors, they explained that it would've been distracting if an energy surge occurred every time Boo laughed, so they simply didn't address the issue. A reasonable course of action, but still a mistake.] (00:37:30)
Deliberate mistake: The 'NNN' news program shows a picture of some animal crackers in Frank's stomach. The problem is, they are still all in one piece and are pretty big. I don't think Frank would swallow them whole.
Deliberate mistake: When her father jumps on the brakes, they lock up, even though the car is supposed to have ABS, which is supposed to prevent this from happening.