When trying to create a new user (either through some Back Office page or on one of the registration pages), a message pops up telling you that a user with this username/email already exists. But when you search for them in Contact Management, no results are returned. How can this be?
The reason is probably due to the account being marked as deleted or expired and by default, Contact Management Search doesn't look for any deleted or expired accounts.
To find these accounts, make sure you click on the Account & Address Options tab and make sure that the Include Expired Contacts and Include Deleted Contacts are both ticked, as shown in the screen shot below. This will extend the search to include all user accounts, whether they are still active or not.