Deliberate "mistake": When Mr. Holland is helping Gertrude Lang with the clarinet, her hands are in the wrong spot. The left hand is where the right hand should be, and the right hand is where her left should be. You can not play the clarinet this way.

Visible crew/equipment: There is a scene where Mr Holland discusses Beethoven with his students and the camera pans over a record player. The camera is reflected in the sheen of the record.

Continuity: Glen Holland owns a Chevy Corvair throughout the film. The car at the beginning of the film is a 1961 model 500 (the base model with bench seats), but at the end of the film it is a 1964 Monza (the top of the line model, with bucket seats).

Continuity: In the scene where Glen Holland hears that John Lennon was killed, he is sitting at his piano and outside the window it is sunny and leaves are on the trees. As he walks home, it is overcast, snow is on the ground, and the leaves are off the trees. Since Lennon was killed in December of 1980, the latter "weather" would be correct.

Revealing: Anytime the camera is on Richard Dreyfuss' hands while he's playing the piano, it is clear that he is not actually playing; his hands are frequently in the wrong part of the keyboard for the sound being produced. At other times, when his hands are moving down the keyboard, the pitch we hear is going up, or vice-versa.