Currently, it's a bit confusing for admins without hideuser right:
- Admin sees EvilUsername
- Oversight sees EvilUsername and is faster than the admin: He blocks EvilUsername with hideuser
- Admin goes to Special:Block/EvilUsername. There is a warning, that EvilUser is "Already blocked", but the block log is empty.
- Admin tries to block user and gets a strange error: "Error: could not submit form" and "<hookaborted>".
Expected behavior: Nice error message saying the admin is not allowed to change block settings as the user has been hidden.