Continuity mistake: Malcolm's seat belt changes from on to off the first scene with him and his mom in the car. When it's a shot from the front it's on, but when it's a shot from the side (from the driver's/mom side) you can see that the part of the seat belt that's going across is now behind Malcolm, instead of on him like it was a second ago. This changes back and forth.
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.
Plot hole: Malcolm mentions how his parents would flip if they knew he was still dating his lying girlfriend. But in the episode before (while in the car with Lois, at the end) he mentions how his girlfriend has him guessing what she wants to do. The way he says it, makes it known that he is still dating her. He says it in present tense. Therefore, they already know for sure, from his own mouth.