I know because it's happened to me. Twice.

Facebook just out to the entire social network, but I jumped on board and published my Facebook Page using the Timeline layout as soon as it was available at the beginning of March. I added a Cover photo, and went through the process of customizing the linked tiles below the Cover photo to with my own pictures.

Fast-forward a week or two. I logged in to Facebook one day to find that one of my images had been replaced, and I had absolutely no idea where the new picture came from. I could almost understand if some Facebook glitch randomly substituted one of my other images, but it seems like a security or privacy concern for Facebook to display someone else's image on my Facebook Page.

I decided at the time that it was just a fluke, so I went back into the settings and replaced the bizarre random images with my own pictures once again. But, a couple weeks later two of my images were again replaced by pictures that don't belong to me, and the same thing happened to my wife's Facebook Page for her business. Apparently, it wasn't just a fluke.

The images that appeared on my were unremarkable. They weren't family vacation pictures, and didn't really seem to be personal at all. It's possible that the Timeline glitch scrambled the data on the Facebook server, and was simply serving up images from other Timelines. But, that's just speculation. It's possible that private pictures were somehow being shared out without permission as a result of this Timeline glitch.