I did have a gun go off all by itself once, a Savage Fox double barrel 12 gauge.
Something broke internally, and I closed the action, the left barrel fired.
Though, that's odd, let me try that again.
Made darned sure my hands were away from the trigger, reloaded, and BOOM.
Relatively easy fix, but it's a damned good lesson why, when loading a double, you should ALWAYS keep the muzzles pointed at the ground and bring the butt up to close it, not the barrels.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.