Francis asks Hal if he can "cash in" his one free pass so that he doesn't get in trouble, with Hal responding that it doesn't exist. However, in "Reese vs. Stevie", he tells Dewey it does exist, and that Francis used his up years ago when he got handcuffed to a stripper pole for trying to steal his tip back from a stripper.
When Dewey is chewing his stuffed animal's ear, it's the left one. In the next frame and different angle, he is now chewing the right one, but in the next frame, is again chewing the left ear and pulls it off with his teeth. The stuffed animal also has both it's eyes for the 1st few frames, but when he pulls the ear off with his teeth, the right eye is hanging by a thread, obviously 2 different stuffed animals.
After Malcolm gets yelled at for throwing a brick through Cynthia's window when both the children are talking, you see Cynthia's hair change from behind her back to on her shoulders to on one shoulder several times between shots.
In the chemistry lab Cynthia pours two liquids into a beaker. In subsequent views, there is less liquid in the beaker.
The pool balls change position immediately after Francis takes the shot. Before the shot, there was a red ball under his left shoulder. After the shot it is on the other side of the table. After Hal talks to him, the red ball is in the original position.