If you use Picard with a non-UTF-8 locale, saving files whose filenames would contain unsupported characters gives an error and fails to rename the file, e.g.
UnicodeEncodeError: 'ascii' codec can't encode character u'\xeb' in position 30: ordinal not in range(128)
UnicodeEncodeError: 'latin-1' codec can't encode characters in position 20-22: ordinal not in range(256)
UnicodeEncodeError: 'euc_jp' codec can't encode character u'\uff5e' in position 35: illegal multibyte sequence
I did see a message when using an ASCII locale (not for the others though), but I think it would be better if Picard could handle it more intelligently and replace any unsupported characters with something else instead of just having errors.
from someone who needed help on IRC: http://chatlogs.musicbrainz.org/musicbrainz/2012/2012-03/2012-03-10.html#T21-54-34-143034