This happenrd to me;
Rem. 870 wingmaster 12Ga. 3"
Cold day duck hunting below zero(broke ice getting onto and off lake). We had a diver down. and swimming away fast. I tossed in a load for one last shot then it was pull anchor and get after it. Well when i shot the last load, I knowingly left the spent shell case in the chamber and cased the gun. we put the chase on but lost. Then upon anchoring and getting set up again i uncased, jacked out the spent case, put one into the open chamber, just as i always do(then two into the mag) when i closed the slide w/ butt on the floor of boat the gun whent off. It was right by my left ear. My son-in-law was looking my way and said "Wow you just about blew your head off" Upon looking the gun over, the saftey was still on, and the shell'sprimer was litely dimpled. at home upon reexamination, the gun was like all guns fresh from field, dirty but not grossly. I thought maybe the fireing pin seezed in the fired position by leaving it with the hammer/stricker in fireed position, but the pin would retract back into the bolt even with hammer down.
Never had anything like that happen again. But i don't carry a gun much anymore as the young grandchildren need watching.
Gbro