To add to the list: Song titles that cannot be fully represented in Unicode. Example (also to plug my favorite band): Magma, a French progressive rock [*] band that's been (on and off) active for more than 50 years, mostly uses their own invented language "Kobaïan" where some of the characters use diacritics that are (to the best of my knowledge) not present in Unicode. E.g. there's an S with a kind of crown on top.

[*] Actually their style of music is unique enough that it's given rise to its own genre, "Zeuhl".