A very few of my files - all demos, so they may have originated from the same tape collector once upon a time - has this tag. I got curious. What it is and what created it?
I found this: https://github.com/myers/pyid3/blob/master/test/test_samples230.py
# EVIL HACK
# don't check files that have a TXXX 'CT_GAPLESS_DATA' frame
# these were written by some buggy software that didn't put the
# unicode BOM on the COMM comment
Not that I am too bothered about the non-gapless tape hiss in between the music, but ... still curious.