Other mistake: When Christopher shows up, he says "tomorrow is Saturday." Wouldn't the girls be at Friday night dinner? Not ordering pizza?
Other mistake: When it is Dewey's birthday Malcom grabs a piece of the cake. It goes to Dewey then back to Malcom and he is chewing, but there is no bite in the cake.
Other mistake: In the second season episode "The Car Salesman", notice the license plate on the real estate agent's truck. It is the same license plate that is later used in the season 5 episode, "The Ski Lift".
Where in the World Is Pookie Stevens? - S3-E2
Other mistake: During the yard sale, in the first shot of Donnie's area where he has all his sports stuff, the sign behind him reads "Donny's Corner". "Donnie" is misspelled here. In all the shots after this one, there are sneakers, or some other equipment hanging over the "y's", hiding the spelling mistake.
Dutch Art and Dutch Courage - S7-E4
Other mistake: At the start just before Michael gets out the car, he puts the handbrake on, but the car carries on moving.
Other mistake: When Viper slides the swords to Jackie and they come to a rest next to him, there is a small paper spider right beside Jackie. The only paper spider in the room, however, is Origami himself, and he is actually too far away at that moment and too big to fit the bill. Guess some of the animators got their depth perception messed up.