In the beginning of the movie, when Charlie is playing piano with his mom, Charlie is playing the bass line (sitting on the left). High, treble notes are then heard, but his mother's hands are not on the keys. Charlie was sitting too far to the left, and would have had to reach across her body to play those high notes, which he doesn't. Therefore, those notes couldn't have been played the way they were sitting and the way their hands were.
The sign on the door for the boys restroom periodically changes from reading "BOYS RESTROOM" to "BOYS WASHROOM" because in Canada, where the film was shot, they use the word "washroom" instead of "restroom".
When Murphy is first introduced in the movie, in the scene where he is selling drugs in the bathroom, his cigarette is nearly done, but when he is giving Charlie a 'Swirly' his cigarette appears as it would right after being lit.
Charlie's hair has been roughly towel dried after a fall into Nathan's swimming pool. At 1:28:51, Charlie says "No, you're her father, and she loves you very much, and you're totally missing it." Between cuts in filming this spoken sentence, Charlie's towel-dried hair changes shape.