In an earlier thread I referenced my story of a 9 in a 40, and a 9 in a 45 and 40 in 45.
The 9 in a 40 is really quite benign when it happens. As the above poster mentioned, it goes bang -- not terribly accurate but will not cycle the slide. My buddy grabbed the 9 ammo loaded it in my 40 and shot. We looked at what happened and said -- OK, that was strange. Realized the mix up and corrected -- no harm no foul -- but I do get to laugh at him from time to time. The 9 in the 45 was ME - ALL ME!!! I grab my Colt 1911 in 9 (or so I thought) it sits in the safe right next to the Kimber in 45 and 2 other 1911s. Get to the range with the mags for 9, ammo, etc... Load mag, shoot one round, notice the very weak report and basically a keyhole in the target, no slide cycle either. Wonder what gives, manually rack, but have to pry the case to eject to find the 2nd pictured case and sit there for a second wondering what the @#$# caused that. -- Then of course I read the side plate and in bold script is KIMBER! -- Did I mention it was a 45 Kimber, not a 9? And of course another buddy is watching me and once we figure out no harm no foul proceeds to laugh at me -- imagine that! Needless to say, I had no 45 mags, or ammo, so that ended that portion of the days festivities.
The 40 in a 45 was another buddy, grabbed a 40 that was somehow mixed in with his open box of loose packed 45 and it went into the gun, same story, weak report, no cycle and out comes item 3. Those all happened within the last 2 years, so it's not like I intentionally set out to create artfully decorated cases. But I do keep them to remind me of what inattention can cause.
But since Walk and others have done it and owned up to it, I feel much better now!
But maybe we have to reconsider having the same gun in multiple calibers? --- naw, what am I thinking?