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Sounds like another case of I forgot it was in my pocket when I sat down just like the iPhones and other self bending phones
A front pocket will do the same and a phone isn't designed to be sat on so hardly normal use
But you would need to either return your device to the place of purchase or contact Xperia care and have your device inspected http://www.sonymobile.com/global-en/support/contact-us/
My Z3 is broken at the power button, too. It is not a miracle at all and yes, I had it in my pocket. What a pity. I think it is the weakest part of the body. My Z1 did not get any harm with the same cover and in the same pocket for almost 2 years now. Z3 only lasted few weeks until the breakdown.
SONY, I don't only want a flat smartphone, I want a solid one.
Glass bends all the time; it's known for its elastic properties.