In between scenes, one of the prop crew (I forget who) was testing one of the guns for a future scene. He fired the gun, and heard a thud of some sort, but nothing happened. He thought nothing of it. For some reason, none of the guns were cleaned between scenes, as they are usually supposed to be.
Filming began on the "flashback", where you see the red/yellow colored scene of Eric coming home to his apartment to find T-Bird's gang attacking Shelley. Blanks were loaded into the gun. The scene called for Eric to walk into the apartment, and startle the bad guys. T-Bird would then turn around and fire at him. A squib (a small device placed on an actor that explodes to give the impression of a gun shot or wound) was placed inside a bag of groceries that Eric was supposed to be bringing home. When shot at, Eric was to fall backwards, and the rest of the scene to be filmed.
Tragically, the uncleaned gun had a lodged shell stuck in the barrel. When shot with a blank, the effect was that of a real bullet: the shell was shot out of the barrel at a high rate of speed. The shot accidentally hit Brandon in the stomach, and he doubled over on his knees and collapsed. Although the scene called for him to fall backwards, everyone just thought that he was over acting. They continued shooting the scene, until they realized something was wrong. He was bleeding badly from the abdomen.
Brandon Lee's real life fiancee, Eliza Hutton, did not make it in time to the hospital before he died. (An eerie mirror of the movie.)
In the end, a terrible mix of mistakes, not any secret plot, took Brandon's life.

