I have had many phones in this front jeans pocket (They all lived) and somehow a slight bend has done this. Very weak, if you ask me we are going to see a lot of this as it seems the phone is not the strongest at all. Time for a unibody HTC I think. I've had it with Samsung's plastic crap.
My phone will be up on ebay if anyone wants an i9500 for spares.
Edit: The gorilla glass is fine, its the amloed behind that cracked.