When joining official DayZ servers (after having left with an automatic weapon in my hands) I sometimes find the gun firing on its own. You are able cancel this by clicking once. If the mouse is left untouched the weapon will fire until the magazine is empty. This happened to an KA-74 on Sakhal version 1.27.159248 Experimental. The KA-74 had a full magazine (30 rnd), PSO-1 scope, Ghillie Rifle Wrap, Polymer Stock, and AK Rail Handguard.
This bug also happened to me previously on a different computer a year or two ago. I had a Le-Mas (potentially sawn off) with a full 25 round magazine and potentially a Ghillie Rifle Wrap. That was the first occurrence of the bug I experienced.