The following exception is occasionally thrown:

E: 08:49:01,364 util.thread.run:64: Traceback (most recent call last):
File "./picard/util/thread.py", line 60, in run
result = self.func()
File "./picard/ui/metadatabox.py", line 527, in _update_tags
new_metadata = file.new_metadata
File "./picard/formats/id3.py", line 648, in new_metadata
copy.copy(self.metadata)
File "./picard/metadata.py", line 360, in copy
self._update_from_metadata(other, copy_images)
File "./picard/metadata.py", line 389, in _update_from_metadata
for k, v in other.rawitems():
RuntimeError: dictionary changed size during iteration

I don't know how to reproduce it, seems to be a timing issues where Metadata gets changed while being read.