About 100 files managed to be created (I think mainly via FTP) with filenames that were invalid UTF8. This was across a bunch of users, and the symptom was that the filename was totally blank.
I've fixed that for all but one file (assumed that the characterset of the file name was the user's default characterset) - and in that case the user had already managed to upload an identical file with the same name so I removed the broken file and sent an email about it.
I've also made the code do that explicit fix during the "Create" path so we won't have this problem in future.
Feel free to discuss in this forum thread.