The best description of a 20/12 accident I've read is in a book called 'The Better Shot'. It describes just such an accident scenario during the action of driven shooting where the unfortunate shooter simply left a 20ga shell in his pouch. It can and does happen.
We had one here locally when a high experience shooter got careless and loaded a 20 in his 12ga 390. It went click. This was during a 5-stand practice round, so no stress or flying birds about. He figured he simply neglected to load it. Nobody questioned the guy or shouted "Stop!" like we sometimes hear on the ranges when such a thing happenes because this guy is so well respected and otherwise careful.. by his estimate he has fired close to One Million shotshells during his considerable years of mostly competition skeet shooting. The 390 barrel was destroyed, but no other damage. If it can happen to him...
My point about the repeater magazine is that it's unlikely, though probably not impossible, for the shell stop (the latch that retains shells in a tubular magazine until it's time to feed) to actually keep a smaller gauge shell in the magazine tube if you try to load one. A small gauge shell 'hiding' in the magazine tube is unlikely.
