I've been browsing this thread because I've also experienced a mysterious break in my screen.
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I took a photograph on the phone while I was inside and there was no sign of damage. I showed people the picture and then proceeded outside to line up for a group photo. Went to take another picture and noticed the cracks over the screen. The people that had previously viewed the photo I took were surprised and shocked that my screen was broken for no apparent reason.
I had been experiencing another problem with my phone over the previous 3 weeks, namely that WiFi had stopped working. I had tried a hard reset and still no joy. Was about to take it into shop for a warranty examination when the screen cracked. Anyway, I took it in and explained to the shop that the phone had 2 problems, the
FIRST being the WiFi not working, the second being the mysterious screen crack. They charged me $52 up front and sent it off to Samsung. I've heard back that Samsung believe that the screen was broken by me and won't fix it under warranty. When I asked about the WiFi they said they weren't aware of that problem but they expect that the warranty would be void because I had broken the screen. I jumped up and down and asked them to look into the WiFi problem first and see if they could fix it or whether it needed a replacement phone. They are yet to get back to me.
The main reason I posted here was to let the forum know that there is at least one other S3 that has had a mysterious crack.
Cheers
Gregg