I'm not entirely certain what's causing this, but I do know what kicked it off.

I'm doing some migrations of the media files between boxes and it seems to have somehow messed up the thumbnails and some cached remote images. However, I've seen no problems with any uploaded files (aside from thumbnail views of those files).

This appears to be resolving itself over time as the server updates contacts and recaches many of these files.

Migrations should be finished in the next day or two and there should be no significant downtime.