Count me as a believer. Glock 17L, rental. I sorta doubt it was new, but I can't say for sure. I tried using the slide release vs sling-shotting, and it didn't make any difference for this gun. First shot was probably 15-20 MOA left. It was easy to see, because all the other shots went through a single ragged hole at what I recall was probably 15 or 20 feet. The first shot always refused to join the party, and it was always 3/4" dead left. 30-odd handguns fired, and this was the only time I have experienced it. Not to say that I can shoot all handguns accurately enough to have easily noticed something like this; with the G17L, I noticed on my first mag, was satisfied something fishy on the second, confirmed on all subsequent mags. After the second mag, I shot off the rest of my ammo loading only 5 rounds and shooting for accuracy, just to re-experience this. It's such an odd thing to see. I should also note, I was a better shot back then, 20 years ago, and to this day I rank that gun as probably the most accurate (in my hands) handgun I ever shot, up there with a particular old West German Sig P220.
I wondered if it was possible that my thumbs were pushing on the side of the slide while cycling the subsequent shots, so it is interesting to see that this can happen in a Ransom rest, too.