[bat119]

In the case of Brandon Lee the prop guys improperly checked the gun

the prop gun, which is a real revolver, was loaded with improperly-made dummy rounds, cartridges from which the special-effects crew had removed the powder charges so in close-ups the revolver would show normal-looking ammunitions.
However, the crew neglected to remove the primers from the cartridges. At some point before the fatal event, one of the rounds had been fired; although there was no powder charges, the energy from the ignited primer was enough to separate the bullet from the casing and push it part-way into the gun barrel, where it got stuck (a condition known as a squib load).

For the fatal scene, which called for the revolver to be fired at Lee from a distance of 3.6–4.5 meters (12–15 ft), the dummy cartridges were replaced with blank rounds, which contained a powder charge and the primer, but no solid bullet, allowing the gun to be fired with sound and flash effects without the risk of an actual projectile. However, the gun was not properly checked and cleared before the blank round was fired, and the dummy bullet previously lodged in the barrel was then propelled forward by the blank and shot out the muzzle with almost the same force as if the round were live, striking Lee in the abdomen