Username already exists but can't be found?

In trying to set up a multisite I've added several users for the blogs I'm establishing. On a couple of them I made mistakes, so deleted them and then added them again.
For some reason, with one particular username, when I tried to put the info in, it says the username is already reserved. I've tried adding it as an existing user, and it says this user does not exist. It's not on the user list for either the particular blog or the network admin.
Can't add new. Can't add existing. Can't delete. What to do?