When I hunt with a 12 gauge, I make sure that all my 20 gauge shells are out of my vest and pockets. It's very easy to drop a 20 gauge shell into a 12 gauge gun and fire it.
That happened a few years ago at a still target shoot with one of the most experienced shooter there. He just took the wrong gun to the line. He fired a shell and did not see anything wrong. Fortunately, several of us heard the shot and knew something was wrong. The shell fired and the entire shell then went into the bore.
Had he fired a 12 gauge shell next, the consequences could have been a disaster either for him or for the shooters next to him on either side. He kind of laughed it off but none of us did.
Thanks,
Clark