If interrupted by a signal (e.g. from ^C), metaflac sometimes leaves a file behind named trackfile.metadata_edit .
Can you please provide me with the full command line you are using as well?
It happens while adding tags. Playing around, I was able to reproduce the problem manually with the following command (run in a directory containing 13 .flac files):
metaflac --set-tag='AAA=dummy' --set-tag='BBB=dummy' --set-tag='CCC=dummy' *.flac
However it does not happen very often.
Here (attached) is a script which reproduces the problem. It repeatedly runs various metaflac commands and then interrupts them after a short random delay. On my system a xxx.metadata_edit file is left behind after about a minute or two. Sometimes other file corruptions occur.
Ideally metaflac should catch signals and always clean up (and leave the .flac file in a non-corrupted state) before exiting.