Did a search and didn't see this one. It's a bug that seems to occur after some pretty specific (and admittedly rare) circumstances.

I did a large dump of unedited photos from my computer to my phone. I then began going in and rotating photos on my phone that weren't oriented properly.

What I found is that after rotating many photos, I'd try rotating again and the photo viewer got stuck in a work loop (little red dots moving across the top of the screen over and over, but nothing more). Windows button still returned me to the home-screen, but the photo gallery then had to be restarted. I couldn't just switch back to it in the task manager and have it right itself. There's no set number of photos this happened after. In fact, it seemingly got more frequent as I got more of the locks.

This is on a newer black 920 (bought in mid december), Portico installed prior to purchase from AT&T, no other issues whatsoever.