Normal I applied a update of MusicBrainz metadata to a large number of files (~20,000), including moving them to a new folder. When it was all finished, Picard showed 0 pending files. I hit Command-Q to close the app, and was prompted "Are you sure you want to quit Picard? There are 6 unsaved files." I wasn't terribly worried about those 6 files so I pressed "Quit Picard", and that moment the app crashed with the following log:
Traceback (most recent call last): File "tagger.py", line 17, in <module> File "tagger.py", line 1583, in main File "tagger.py", line 751, in run File "tagger.py", line 738, in exit File "tagger.py", line 635, in run_cleanup File "coverart/image.py", line 111, in delete_file KeyError: 'bbc4a102e5448a34b353f425e906686e32b26e223dbb6cf09cca8a2c8ab9f4e5e9e2d74aa4490a130aaabac6904e3b2b29116c36cc27b36767bd7607b09f0def'