Actually had a client do this:
1. Create a user
2. Renamed the user, but accidentally used the full domain (email@example.com)
3. It creates the user as firstname.lastname@example.org@domain.com
4. You cannot delete or remove this user
5. Opening the user to modify only opens a blank user that has all appearances of being an AD Integrated user with no information filled out. You cannot make any adjustments since there is no user information present
The only way I found to remove this is to rename the user folders and then remove the users from the accountlist.xml and restart the SM service.
Its nice that SM will not allow you to create a user with the @domain.com in the user name field at the start. That prevents this from happening, but the rename does allow you to create this situation and I would be afraid someone might do it to a live account and thus force a laborious restore process.