This is a somewhat incomplete report because I'm not really sure what happened. When trying to block the account named Ks0s got STUNG due to INCOMPETENCE of CIDR\17 block on enwp, Oshwah encountered some trouble with the Special:Contributions UI thinking it was an IP range, and ended up trying to block via the API.
Somehow they blocked https://en.wikipedia.org/wiki/Special:Contributions/Ks0s_got_STUNG_due_to_INCOMPETENCE_of_CIDR (a nonexistent account, has ipb_user=0 in the db), and the actual account https://en.wikipedia.org/wiki/Special:Contributions/Ks0s_got_STUNG_due_to_INCOMPETENCE_of_CIDR%5C17_block
These are blocks 8081998 and 8081999 in the enwiki database. I'm not sure if there's something wrong with how we handle usernames with \ in them, so marking security out of caution. There does seem to be an API bug in that it was possible to block a non-existent account?