Sadly I have had it happen to me on a bolty, in my CA m24 to be exact.
IIRC it was all shooting fine then suddenly after firing off a shot the bolt pull got a little slack and not as stiff when I cocked it, I then fired it and the bb acted as if it had been made of lead, cocked it again then the snapped section of spring got jammed between the piston and cylinder side scratching the crap out of it, luckily after the O ring reached its maximum travel.
I put it down to old age and metal fatigue :shrug: