Problem 1: When mp3tag can't write to the file (or at least, that the message it is giving me.), it leaves a .tmp file lying around.
Problem 2: I'm getting the error "cannot be opened for writing" on certain files I have, even though they clearly are writable. I know that because i checked the permissions are exactly the same as other files that it works with, and I also tried writing to the file myself, as well as copying it to another filename. So I assume "cannot be opened for writing" is actually some other problem with the file. Needs a better error message.
Problem 3: I don't know why this particular movie file can't be edited. I've got 100 files from the same source, 2 of them don't work with mp3tag. Maybe there is something wrong with the files. I can't tell what. A bit tricky to supply the file for diagnostics since it's over a gigabyte.