Today I ended up with a bunch of blank files. The server (SFTP) ran out of space and Yummy FTP gave an error about not being able to save the files that I was editing remotely in TextMate. I should have kept the files open, but instead I closed them and tried to re-open the originals, and they were empty.
I'm not sure what Yummy FTP should do in this case, but some kind of warning that made it clear what had happened, or kept a backup of the original files might be useful. I can see that copy/renaming rather than overwriting might make things slower though. And it's my fault for not checking everything into version control yesterday, really.