Continuity mistake: When Lois and Kitty Kenarban (Stevie's mom) are at the table in the restaurant, while Hal and Abe sit at the bar, things on the table change position or disappear and reappear between shots. Same thing happens at Malcolm's table with Reese and Stevie.
Continuity mistake: In the scene where Lois and Hal tell Malcolm he is going to the "special class", Hal notes that the pitcher of iced-tea he makes is always empty, and says, "Look at that - gone," before placing the lid back onto the pitcher. Yet, when it cuts to the wide shot of the table, the pitcher lid is by Hal's elbow. (00:17:50)
Continuity mistake: When Julie passes through the kitchen to get Malcolm, Dewey and Egg are in there cleaning the floor they have suds all over the floor, When Julie runs back through after seeing Malcolm in his underwear she slides across the floor, The floor is still completely covered in suds but when she gets up if you look in the background you see the floor with hardly any suds anywhere. She could not have soaked up all the suds when she slid across.
Continuity mistake: When Lois is in the bath tub soaking after getting upset about the family wanting to go to the funeral. Dewey comes in and ask for juice. When they are on Lois(front view) the suds barely come to the top of her breast. When the show Dewey (side view of Lois) The suds are all the way up to her shoulder.
Continuity mistake: When the Tarantula is put on Hal at the zoo, the shot is only of a yellow shirt with the tarantula crawling on it. When the shot changes to where they show the whole Hal, you can easily tell that the tarantula is fake - it is larger than it was and is in a different spot on Hal's shirt.