In the ACP - if you find a user and then attempt to rename them, the error dialog come up if the name is already taken. However, once you dismiss the error dialog, the form page now shows as "Edit User 'taken-name-you-tried-to-rename-it-to"....
The proper display should revert to their original name before you tried to rename.
This does not stick - so therefore the only bug is with the improper display, but it is confusing as it makes you think the user has been renamed.
If this was a returned value from the save then I would call it a bug since the username didn't take. But it's not a save response, and it does leave you with this little error next to the username field:
I think it's clear enough. But I will move this to suggestions.