Symptoms
Task Drop database user for MS SQL user fails with error:
Login is aliased or mapped to a user in one or more database
Cause
This error is returned by the MS SQL server itself, and can be reproduced by running query
exec sp_droplogin '<USER_LOGIN>';
It may be caused by a bug in MS SQL: http://support.microsoft.com/kb/223999
Resolution
Please refer to Mictosoft article http://support.microsoft.com/kb/223999 for resolution, or try to use Enterprise Manager utility to delete this user. After that, rerun the task.