Reportedly, publicity for this picture during theatrical release declared that this movie detonated the largest ever (at least at the time) controlled explosion in Los Angeles. Apparently, the explosion utilized five tanks of butane fuel and nine pounds of black powder. The object being blown up was a fifty-years old run-down skid-row hotel which was actually about the same age as the movie's star, Charles Bronson.
When Bishop pins up the papers that he gets in the special delivery, he puts the pin in the top middle of the paper. In the next shot, the papers are pinned in both the top left and right corners.
Arthur Bishop: [When Arthur is explaining to Steve about the rules in the martial art scene]
They won't hit each other or one of them would dead and that's against the law.
Arthur Bishop: [voiceover as Steve reads note]
Steve, if you read this, it means I didn't make it back. It also means you've broken a filament controlling a 13-second delay trigger. End of game. Bang! You're dead.