I thought I was the only one who found those! I've also found .40 S&W that have been fired in .45 acp, and 357 Sig that were fired in a .40 S&W barrel, which wouldn't cause any gas leakage, but the accuracy would greatly suffer.
In my experience, the two most common are the .380 in a 9x19 barrel, and 9x19 in a .40 S&W barrel. A couple of months ago, some rocket scientist at our range put a whole box of 50 9x19 through a .40 S&W. I didn't see him do it, but it was in the brass I bought from the range.
In most semi-auto pistols, the extractor will hold the case against the breech face firmly enough to allow the primer to ignite. That's why they print the caliber on commercial firearms and on the boxes of commercial ammunition, in the hope that someone will actually read it.
Hope this helps.
Fred