Cracks can certainly originate due to microscopic defects in the polycrystalline grain structures in glass. With the volumes Apple ships out, its not inconceivable there could be a handful of cases that slip out..
I don't disagree that this couldn't happen. However, Let's not have any false modesty either, when you carry a phone that's constructed of glass in your front pocket and pressure is applied at a specific angle with just the right amount of force, glass can crack even if the OP was not aware of it, especially being a case was concealing the crack to begin with until they figured it out later on. Applied pressure and stress with the glass over time might have been something that started as a small crack and grew larger.
so I wouldn't dismiss the OP outright. Apple may choose not to believe him, but I see no reason to not believe him either....
You are taking the word '
Dismissing' out of context . Dismissing would be if I said I didn't believe the OP based off their rendition. I believe/acknowledge the OP doesn't understand how the crack occurred, however, I simply offered my theory that a crack doesn't just inexplicably happen without something of force causing the crack to begin with, which if the phone was in their possession, it is their fault, which the OP claimed it was not their fault. Especially if there is no evidence proving how the crack actually happened in the first place. But it's possible it could be a manufacturing defect, even though I think that's a rarity in most cases.