The scene when he was on the table being shot at, unfortunately one of the guns had a real bullet in it. Lee falling in that scene was actually him being shot dead.
Further to what Easy10 said, here's what IMDB has to say:
'Lee, Brandon' died during a mishap on the set. A scene required a gun to be loaded, cocked, and then pointed at the camera. Because of the close-range of the shot, the bullets loaded had real brass caps, but no powder. After the cut, the propsmaster (not the armsmaster - he had left the set for the day) dry-fired the gun to get the cock off, knocking an empty cartridge into the barrel of the gun. The next scene to be filmed involving that gun was the rape of Shelly. The gun was loaded with blanks (which usually contain double or triple the powder of a normal bullet to make a loud noise). Lee entered the set carrying a bag of groceries containing an explosive blood pack. The script called for Funboy (Michael Massee) to shoot Eric Draven (Lee) as he entered the room, triggering the blood pack. The cartridge that was stuck in the barrel was blasted at Lee through the bag he was carrying, killing him. The footage of his death was destroyed without being developed. Lee is the son of martial arts legend 'Lee, Bruce' , who died in mysterious circumstances before completing Game of Death (1978). See also Dragon: The Bruce Lee Story (1993).