Other mistake: When 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 mistake: 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.
Trivia: All the American Sign Language used in the movie is accurate. When Iris tells Cole to open the window, Cole's response is not translated and she says "don't bust my butt." What he signed was, "Tell him to open the window himself."
Glenn Holland: You work for 30 years because you think that what you do makes a difference, you think it matters to people, but then you wake up one morning and find out, well no, you've made a little error there, you're expendable. I should be laughing.
Glenn Holland: Well, congratulations, Gene. You've been looking for a way to get rid of me for 30 years, and they finally gave you an excuse.
Vice Principal Wolters: You know, I'm not as popular as you. I'm not anybody's favorite anything.
Glenn Holland: That's because you're the enemy, Gene. You just don't know it.
Question: Sometime during the film the show the passing of several years by showing images of what happened during these years. I think it is at the time of the Vietnam War. A song is played in the background. Does anybody know what the name of that song is and whom by?