When Maurice pulls down Brian's pants, he gets embarrassed and yells at Maurice saying that no one ever sees him in his boxer shorts - not even his mom. But, on the first night Brian follows Maurice to Monster World, he ran around in his boxer shorts all night.
When Brian and Coleman are fighting at the school you can clearly see Brian knock Coleman's hat off and see the hat fall. However, when the Principal is walking towards them and they're shown fighting again, the hat is back on Coleman's head.
Brian takes a Polaroid picture by holding the camera in front of himself while in the classroom with Kiersten. Later in the movie, the photo is on Kiersten's nightstand but he is posing with his arms folded.
When glen is flipping through the channels without the remote he puts on a Celtics game and then phillies games where harry kalas announcers mike schmidt's name then when glen opens the window and asks brian and eric if they seen the remote because he wants to watch the sox and celtics games however inter league play did not begin until 1997 so their is no way the redsox could be playing the phillies.