Continuity mistake: Season 6, episode 7: "Hal Sleepwalks"; At school, Dewey stops the shouting match between the two opponents. In the shots facing Dewey, the boy's arms are held way up high in front of him, but in the shots facing the kids, the boy's arms are down at his sides.
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Continuity mistake: Episode 16 Season 6 "No Motorcycles." Francis calls Hal out to the driveway where two motorcycles are parked. In the first camera shot the motorcycles are parked side by side with the front wheels turned in the same direction. All subsequent shots of the motorcycles in the driveway show them in a different position, one angled behind the other, with the front wheels turned in opposite directions.
Continuity mistake: In the episode where Lois takes revenge on the four girls who pranked Reese, Lois and Reese use a makeshift catapult to launch paint balloons at one of the girls and her date. The first balloon hits the girl, covering her left shoulder with pink paint. In the shot where her date gets hit with a balloon, her shoulder is no longer covered in paint. Some of the paint blotches change size and shape on both her and her date throughout this scene as well.
Continuity mistake: In the episode "Malcolm defends Reese" Hal shows Dewey that he fixed the microwave so that it works with the door open. There is a red cloth inside. Hal then shuts off the microwave and when he and Dewey start talking the cloth changes position from shot to shot.