I must confess , several years ago hunting quail and pheasants
conditions were cold, driving sleet ,miserable, birds were holding tight and alot of them, dogs were working well,
there was alot of shooting. at the end of the day I noticed my gun had a bulge in the right barrel just forward of the chamber, I remembered a point in time when I pulled up on a bird and the right barrel clicked so I fired the left barrel,
dropped in two more shells and kept hunting but did remember acquiring a major head ache at about the same time.
I had a 20 ga shell in my vest and in all the excitement not taking my eyes off the dogs on point I dropped a 20 ga shell in the gun and then after the misfire thinking in all the excitement I forgot to drop a shell in the right chamber I dropped in a 12 ga on top of it.
I did not go back to try and find the 20 ga shell but what I believe happened was the 12 ga shell fired and detonated the primer on the 20 ga and the whole business left the barrel.
The gun was a 12 ga DHE high condition Parker with fluid steel barrels , she held together , but I had to have it sleeved, Kirk Merrington did the work and it balanced the same and you could not see the seem.
and Yes a lesson was learnt. Insure your vest has only one gauge shell in it EVERY time you head out.

And if Joe shows up to comment on this, thats fine,
I deserve it !