Looping Lines: The entire movie was filmed without sound, and so virtually all the actors had to loop their dialogue after filming was complete, with a couple of exceptions:
Kien Shih (Mr. Han) spoke no English, so his voice was dubbed by veteran Chinese-American actor Keye Luke (of "Kung Fu", "Star Trek", "Gremlins", and "Charlie Chan" fame).
When the extended Cantonese/Mandarin versions were released for the first time in English in 1998, some extra dubbing had to be done, because no English dialogue existed at that time for those scenes. One of the scenes involved Roy Chiao (Shaolin Abbott) and Bruce Lee. Chiao was still alive (he died shortly thereafter), and was able to dub himself, but Lee's voice was supplied by Bruce Lee biographer John Little. Luckily, Lee's real voice was left alone for the scenes that originally used it.
The Other Marty: Jim Kelly replaced Rockne Tarkington, who quit the film three days before production was due to star because he thought the pay was too low.
Throw It In: In the scene where O'Hara (Robert Wall) is beaten by Lee, Lee delivers a flying kick to O'Hara. Wall and Lee had decided that Lee should deliver a real flying kick to add authenticity to the scene, as Wall knew how to take the hit. They had not planned for wall to go flying back into the extras, knocking them all over and actually breaking one's arm.
Unintentional Period Piece: The leisure suits, turtle necks, the funky music, Williams' afro and manner of speech along with mentioning that the Vietnam War was only a few years ago, all point to this movie being in The '70s. Also, this movie is mostly responsible for kick-starting the kung-fu craze in the US during this time.