The sound is also bugged when this happens, the shot is very quiet, but still agroes all the zombies.

The first time it happened was with a bk break action rifle. The second time it happened was with an sk semi automatic rifle. My finger was nowhere near my right triggers. Curiously, the sk semi auto rifle fired twice. It almost seemed as if my double tap triangle input(which unloads a weapon) was somehow scrambled and read as the firing input.

Ruins stealth for me.