when this happens I also get (after editing some other fiels and trying to save) a "file has been modified by another application" followed by a "wrong file format: the file doesn't appear to be a valid aiff (or flac) file" alerts :-/
EDIT: I can now say it happens (almost) everytime :-/
- remove existing artwork and save
- add new artwork, save and remove tracks from Tag (if saving takes a "pretty long" time... the issue will show)
- re-add tracks and... artwork is corrupted
to fix:
- remove (corrupted) artwork and save
- re-add artwork and save (saving is considerably faster)
files are AIFF and artwork is JPG edited using Pixelmator
EDIT 2: oh... it also happens, when the issue occurs, that selecting all track together artwork doesn't show, but selecting each file one after the other does :-/
and sometimes, when re-adding tracks (after first two steps above) Tag says "not a valid aiff file" but... quitting Tag and re-adding those very same files... all is fine
also... *please* Stephen: in alert dialogs, *PLEASE*, add an "Apply to All" option instead of popping-out the alert for each and every file

(try editing a 90 tracks album, eg "A Tribute to Gustav Leonhardt", and if something goes wrong you'll see! already asked this when Tag was in beta, remember?

thanks
EDIT 3 (and hopefully last :-p ): seems the issue is more prone to happen if one adds artwork and changes "type" (eg from "Other" to "Front Cover") at once without saving in-between