Factual error: After Robert De Niro awakens, Robin Williams takes him on a ride around New York and Robert is amazed by all the surroundings. The movie is set in the early 1970s, but you can see plenty of vehicles around from the time that the movie was filmed (1990). Notice the square headlights that originally appeared in the late 70's.
Continuity mistake: When Dr. Sayer and the group of patients are in the dance hall, he is sitting at a table with a drink. He picks up a cherry that is lying on a napkin near the drink and eats it. One of the female patients asks him to dance, and as he gets up the cherry has reappeared on the napkin.
Trivia: During the scene where Robin Williams (the doctor) has to restrain Robert DeNiro (the patient), Robin accidentally broke DeNiro's nose. DeNiro insisted that they continue with the filming until the scene was completed because he knew, from prior experience, that he wouldn't be able to film for at least a week, until the swelling and bruising went down. And DeNiro actually was glad it happened because it corrected the bump from the prior broken nose.