Revealing mistake: In the scene when the aliens first enter Valerie's house, Mac and Zeebo find the fish tank and eat one fish each. The shot turns to Valerie's stunned reaction then back to the aliens, who are smiling widely, pleased with their stunt. If you look at Mac's teeth, you can see they are bright white up to his first molar, which is a give-away at his fake white fronts.
Revealing mistake: Right before Dodger walks into Georgette's room in search of Oliver, she talks to herself in the mirror. But at one point the reflection moves when she doesn't. (00:43:10)