The scene where Roman Polanski slits Jack Nicholson's nose was extremely complex to film, and the two men involved got so tired of explaining how it was done (by using a specially-constructed knife with a short hinge that would be safe as long as it was handled VERY carefully) that they began to claim Nicholson's nose was actually cut.
At one point, Roman Polanski and Jack Nicholson got into such a heated argument that Polanski smashed Nicholson's portable TV with a mop. Nicholson used the TV to watch L.A. Lakers basketball games and kept stalling shooting.
The film's enigmatic title is a metaphor for moral corruption by unseen forces. Throughout the film, Jake Gittes refers to his time as a police officer in Chinatown, where "you can't always tell what's going on." In Hollywood, the movie's line, "Forget it, Jake, it's Chinatown," has become a mantra for those who have been burned or snubbed by the entertainment industry - the implication being that it's better to "let it go" than make an issue of it, because that's just how the industry works.