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: Reese is playing a Game Boy without a game in the slot.
Halloween Approximately - S2-E2
Other mistake: In the scene where boys are loading eggs into their slingshots, one of the eggs misses and falls on the roof, but doesn't break. A real egg would have broken.
Other mistake: At the beginning of the episode when they are driving in the car, the door is opened to make room for the camera outside but the opening is visible in shot. (01:49:00)
Other mistake: The book Reese is sawing through in the background, while Malcom is talking into the camera, is already cut in half and the saw is not sawing anything. Then they cut to Reese about 7 seconds later as he actually saws the book in half. (00:02:13)
Other mistake: In this episode Malcolm's new teacher begins a league table for the Krelboyne class, and since Malcolm shows that he is better than the whole class, the teacher creates a new highest rank for him which he calls "1 to the 11". However mathmatically, "1 to the 11" is still 1, so it seems rather strange that the teacher would go the trouble, and that none of the class would notice this.
Other mistake: When Reese lights the fuse for the fireworks, Malcolm says the fuse is timed for 20 seconds. When the diving board collapses on Reese, it is around 30 seconds after the fuse is lit before the fireworks start.
Other mistake: Lois wins the truck, which is never seen again.
Suggested correction: Just because we're not told what happened to the truck doesn't mean a mistake occurred. There's a number of valid reason we don't see it again (especially when only 3 more episodes were produced in the series).