Revealing mistake: In the scene where Martin Brundle has transformed into the fly he kills one of the guards by breaking his neck. After this he throws the body towards a door where another guard is standing - the body is a plastic dummy as it flies across the room with no effort at all and when the other guard behind the door checks the body you can see a plastic yellow face which is supposed to be the guard that was killed.
Trivia: In one version of the script, Martin finds another videotape of his father Seth. This tape would show Seth (who is in his part-fly form) explaining the "cure". This scene would have required Jeff Goldblum to be in make-up for five hours, and he didn't want to go through that, so he dropped out, and the scene was cut from the script.