Just to recap the 3 most talked about occurrences of camera glass breaking that I remember on this forum:
1. JerryRigEverything: A 'reviewer' that intentionally damaged the camera glass before it cracked. Shocking result. This is the same guy that blasted Corning and Samsung over the Gorilla Glass used in the Note 7 saying it wasn't as scratch resistant as they claimed when in fact it was his hardness picks that were defective.
2. The other thread on here with the lady that pulled her phone out of her purse one day and noticed the glass broken. Turns out, as stated in a separate thread, the phone was actually dropped into the bottom of the purse with countless other things where it could have easily been damaged before cracking.
3. This thread. User admits to carrying 4 keys in same pocket with the phone. Any or all of those keys could have easily scratched the glass causing it to crack.
So we have 1 definite and intentional damage to the glass before it breaks and 2 occurrences where the probability is high that the glass was damaged in some way before it cracked.