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We had a student in class one day that was having trouble with his .45 not firing during the first set of shots. I went over to him and watched as he started to fire the second set of shots. The fire command was issued and when he pulled the trigger I saw a brass colored flash come out the end of the barrel. When the cease fire was issued I went down range about 10 feet and retrieved an intact 9mm round. Luckily the primers on the rounds were not being hit and the firing pin was just shoving the round down and out of the barrel. When I showed him the round, he had a sheepish look on his face. He realized that his wife had shot in the group before him and he had just reached into their range bag and pulled out a box of rounds, not looking at the caliber.

Moral of the story, if you are shooting different calibers, keep the ammo separated.