I have always used my iPhones without any cases or protectors (since iPhone 4s, every model). Never dropped or anything, I am very careful with my devices. Never had any scratches or scuffs, though I never looked for micro scratches on screen under direct light.
For the first time ever, I dropped my phone last night on to a concrete pavement (6s, space gray). I was holding shopping bags and it slipped from my hand (how silly of me), landed screen first, bounced and rested on its back. I was sure it is done for good, I never get Applecare+ or stuff, but amazingly, not a single scratch apart from an extremely minor scuff at the bottom left.
Absolutely no scratches on screen and body, and yes I checked under direct light. I am simply amazed and delighted, never knew it could come out scratchless or without a blemish from such a drop on concrete.
Lesson learned: need to be more careful, still going without cover or cases

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