When I de-cosmo'd my "new" Wasr-10 I didn't quite get the cosmoline out of the bolt/firing pin assembly.
I took it out and started shooting at my range , when the buzzer rang i removed magazine , ejected live chambered round and picked it up.
*** It had a clear dimple on the primer! ****, I put the gun away for the day.
At home that afternoon I discovered the firing pin was not freely floating in the bolt group thing and was in fact gummed up and protruding far enough to touch the primer. I believe I was within a few thousandths of an inch from chambering the gun and having a full auto runaway condition.
You may want to give that bit a little more attention than I did.