I just started using Lightroom 5 and the following thing has happened to me twice within an hour.
Normally, I first rank the photos and then tweak a bit the 4* photos before I export them.
So I go through every file at least once, if not twice. As I was about to assign a keyword to the attached image, it suddenly turned into what you can see - it's been a totally normal photo. The weirdest thing is that the original RAW image is damaged, too. I did a bit of digging around and in theory, Lightroom has Read Only access to the original RAW files - yet the original file is corrupted, too.
Any guess what the cause might be? I'm using an older Canon 1Ds Mk III.