libid3tag. If only anybody had told me this: I was just investigating why some ID3 tags just aren't written for some MP3s. I stumbled upon this piece of code, hidden deeply in the guts of libid3tag:
/* hard general case: rewrite entire file */ /* ... */ done: return 0;
To those not fluent in C++, this means that libid3tag's implementer chose to just forego the hard case and leave that as an exercise to the interested reader. Grrr. :barf: So, I guess it's going to be TagLib by Scott Wheeler.