I watched it happen once, and was standing beside another shooter when it happened, but wasn't looking straight at the gun.
For the second time in this thread...the more detailed version.
I was talking to a guy who was warming up for a plate match one Sunday afternoon. His wife was shooting while he gabbed with me. She had just run a plate machine in rapid-fire, and...hearing the dingdingding...started watching her more closely. Good shootin', said I.
On the next rack, she dinged two plates. On the next shot, I watched the gun cycle..watched the empty case fly...but no plate fell, and there was no strike in the dirt. I yelled for her to stop...but she couldn't hear me, and pulled the trigger. The gun locked up, and she thought it had misfired because there was no report and no recoil.
I was able to wedge screwdriver tip between the slide and hood and get the slide open enough to see that there was a case in the chamber...but not far enough to remove it.
At first, everybody thought it was a live or misfired round...but the screwdriver would only go about an inch deep in the muzzle...so I knew that it had fired at least one of the rounds. I offered to get the gun apart back at the shop, but the guy said he'd take it to Tommy Abernathy the next week, since Tom had wrenched the guns for'em.
I called Tommy mid-week to see what he had found. There were two bullets in the barrel...which was split from the third lug to the muzzle at 9 and 3, with a bulge where they met.
One more time...I was looking straight at the gun when it cycled, observing her technique...so there was no mistake, and there was no chance that the lady could have hand-cycled the gun.
35 years of owning, 25 years of wrenching,
and I've never seen this occur.
I only saw it for the first time..actually saw it...about 3 years ago. Heard of it. Had people swear to it, but doubted it, saying that they'd likely done an automatic malfunction drill, but never actually watched it...until that day.
The other time that I was on hand, the guy said that he didn't hand cycle it...but because I didn't actually see it...I had to be skeptical, but now I don't doubt it any more. I know that it can happen. Not a common occurrence...but it can happen.