Happened last year on a pheasant shoot.An older chap put a 20b shell in a 12b chamber.The 20b shell dropped through the chamber and lodged in the barrel he then thought he had a misfire and put a 12 b shell in the chamber.Squeezed the trigger and boom the barrel exploded taking his thumb with it.
To be fair the guy never whimpered once but was very embarrassed,his fore end wood ended up in pieces and the fore end iron was found embedded in a near by tree.We looked for his thumb as there was a chance it could have been re attatched,unfortunately we think his dog ate it.He made a full recovery and still shoots with us,however we do tend to stand well away when he chambers a shell.
Thanks for your concern...