[Dibby053]

The short-lived country of Serbia and Montenegro got assigned .cs (from Crna Gora - Srbija) which previously belonged to Czechoslovakia, but it seems it was never used (they kept using .yu).

https://en.wikipedia.org/wiki/.cs

[mr_toad]

Reusing country codes seems like it would annoy a lot of DBAs, not just people involved with the domain name system.