The kid who tells D-Fens how to use the rocket weapon could have learned the detailed instructions from watching Beverly Hills Cop II (1987).
The man who is yelling about being "not economically viable" in front of the bank is wearing the exact same clothes as D-Fens. Even the tie pattern is the same.
The stripper for Prendergast (Robert Duvall) is named "Suzie". When she starts dancing, someone says "Susie Q", a reference to the Creedence Clearwater Revival song used in Apocalypse Now (1979), which also featured Duvall.
In the scene in which the drive-by car crashes, there is a mural of Christ on the wall. This same mural is also present in Colors (1988), also starring Robert Duvall, in the scene in which "Pac Man", played by Sean Penn, spray paints a kid's face.
Every studio in Hollywood turned down Ebbe Roe Smith's script. Producer Arnold Kopelson was getting to the stage of considering cable TV when Michael Douglas came across the script and pronounced it one of the best he'd ever read.
Bill Foster never uses the same weapon twice (aside from the baseball bat and Tec-9)
Detective Brian (Steve Park) says that he can't translate for Mr. Lee (Michael Paul Chan) because he is Japanese and Mr. Lee is Korean. In real life, Park is Korean and Chan is Chinese.
Prendergast sings "London Bridge" with his wife on the telephone. The same tune is played by the snow globe that D-Fens buys on Wilshire. The lyrics to the song are "London Bridge is falling down, falling down", a reference to the movie's title. Prendergast also planned to retire to Lake Havasu, Arizona, where the old London Bridge was moved to in 1971.