When trying to create a new account that happens to start with an interwiki prefix, the prefix is dropped, and the account is created with just the suffix as the username.

Steps to recreate:

1. On a test wiki, log off, go to Special:UserLogin, and then click on the blue "Join foowiki" button to get to the signup screen.
2. Enter the username "Wikipedia:John", choose a password, and click "Create your account". (This step assumes there is no "Wikipedia" namespace on the test wiki, and that "wikipedia" is a registered interwiki prefix.)
3. The account is created with the username "John".

Expected behaviour: Account creation is prevented with an error message that "Wikipedia:John" is an invalid username.