If an existing username is used it depends on what settings are sent to the API. It can either just return a failure or try for X times to find an available username, eg. username2

@ Darkmage, is it possible the account was registered as username2, username3 etc? Try checking the most recent members on the forum or (if the API told it to send it) your welcome e-mail.

@ Dwight, most strange. Glad we've sorted how they're registering though. I haven't changed anything on the registration API recently other than add a load of tracking Maybe the system is so clever it debugs and fixes itself