I'm pretty sure I've had at least one squib in a GP-100. I was shooting steel and round 4 of 6 didn't make any noise. My brain caught up to my trigger finger only after I'd already launched rounds 5 & 6. At the time I hoped that it was a light strike (very normal with the ammo I was using at the time, IIRC), but all primers were popped. If it was a squib, it left no damage to the gun that I've ever seen.
If I'm remembering it correctly, the barrel was plugged with something like 17 rds of a popular force on force round. A live round of 5.56 was then fired behind them and Poof!. The fault lay with the instructors in this case, not the shooter. Many of the accidents I have been around for were similar in nature, that is, someone decided to switch a gun back and forth between force on force ammo and live ammo. Nothing good ever comes of that, and dedicated sims guns are the only smart way to go.
That's my only real concern about squibs (not that I lose sleep over them). Bullseye shooting for fun is fine but if I'm running USPSA stages or something 3 or so bullets are gonna get fired before my brain recognizes an oopsie. With my shotgun example I just assumed it was a jam and cycled the round out; only after the RO yelled STOP many seconds later did I realize what just happened.
Semper Gumby, Always Flexible