I have the same problem. Few weeks after my purchase I coincidentally found that C shaped crack near the microphone hole. If I had dropped it pretty badly I would remember. Everyday use shouldn't have caused such a crack. But the crack didn't affect the usability of the phone.
Horrible thing is, yesterday my iphone went into an endless cycle of rebooting. When trying recovery mode, I got the 4013 error.
When I took it to the Genius, he mentioned that the 4013 was caused by a hardware problem and that it is covered under the 1 year warranty.
BUT, when he saw the C shaped crack on the lens cover, he stated that because of that, they will need to charge 549 USD to fix it....
It's pretty funny because on the invoice it says $0 for 4013 error repair, but $549 for back glass repair.
Now I understand how the whole 1 year warranty scheme works.. they charge you for a problem that actually doesn't need to be addressed but state somehow it "must" be fixed order to fix another problem.
Pretty upset but at the same time relieved. I was planning to switch everything to apple (pc->macbook, tablet->ipad, bluetooth earphones -> airdrops).
Everything that Louis Rossmann said about Apple is now making sense..
No wonder so many people recently have been switching back to PC/windows/Android.