I've removed the section about offering free rename tokens to usernames that begin or end with underscores. After almost three years, we're not going to offer this option anymore. It's just time to expire the offer. Nothing's broken with these usernames. People with a username that begins or ends with an underscore just use a different default URL. If they'd like to change this, they can purchase a rename token the usual way to change their username.
For posterity, here's what the bit I took out used to say:
Underscores (0 rename tokens): If your username begins or ends with an underscore (_), you may not be able to use LiveJournal's standard journal URL. For this reason, you have the option of renaming your journal once for free. If you would like to take advantage of this offer, please open a support request while logged into your account.