Continuity mistake: When the van stops and Fish's body is sent flying away, the position of his body on the ground differs between shots.
Continuity mistake: In the "playing bars" music montage scene, there is a shot where Fish and Curtis high five. It's a shot from their final concert that hasn't happened yet. Fish's hair is already short.
Continuity mistake: When ADD is getting ready to go back on stage after Vesuvius fails, the keyboardist doesn't have his hat on because he threw it into the crowd. Then, when they're shown playing after re-taking the stage, his hat is back on his head.
Continuity mistake: When ADD is playing their last song before Vesuvius takes the stage, the camera switches to a long shot and the lead singer is suddenly playing guitar left-handed (everything might be reversed, I only noticed the lead singer).
Continuity mistake: During the performance at the prom, when Fish is ruining the slow song by playing the drums at fast tempo, the black stripe on the back cymbal behind him keeps changing positions many times throughout the song.
Continuity mistake: When Fish is talking at the coffee place with Curtis, the glasses on the table keep moving around between shots.
Continuity mistake: While in the bus stop, Fish looks at the Rolling Stone magazine that a man is reading. The wide shots show the cover folded, but in the close-up it's straight.
Continuity mistake: When Fish is on top of the van and slides towards the windshield, his hand and the drumstick he holds are either against the windshield or away from it, depending on the angle.
