Recently I had an issue while dropping an orphaned user for an obsolete login. I was unable to drop the user and it failed with the below error messages.
Msg 15421, Level 16, State 1, Line 1
The database principal owns a database role and cannot be dropped.
Msg 15138, Level 16, State 1, Line 1
The database principal owns a schema in the database, and cannot be dropped.</code