An invalid parameter or option was specified for procedure 'sys.sp_change_users_login'

Ran into this one today. Restored a database with a local user
account and the SIDs didn't align. Simple matter to fix, run
sp_change_users_login with auto_fix and I'm done.
*clickety*clickety*click* Boom

"
EXECUTE sp_change_users_login @action = 'auto_fix', @UserNamePattern =
'SQL_USER'

Msg 15600, Level 15, State 1, Procedure sp_change_users_login, Line 207
An invalid parameter or option was specified for procedure
'sys.sp_change_users_login'.
"

I ran this a few days ago against the same restore and it worked.
Same command even since I'm lazy and have a cheatsheet of infrequently
used but needed TSQL. A quick lap around google found similar people
having problems but no resolution. Invalid parameter... I tried
changing the case of auto_fix to all lower, all upper but no such
luck. I tried it by explicitly specifying the missing parameters to
no avail.

Being a simple man and not well versed in SQL Server users,
principals, etc I decided to try the simple things first. When I ran
the original command, I was in the sales database. I changed into the
master database and reran. Low and behold, it worked. Where it gets
weird is the command doesn't throw the invalid parameter or option
error if I run it against the parts database. I'm sure once I get
versed in securables in SQL Server, this will all make sense but for
now, I'm documenting it so I remember.

Reference
http://technet.microsoft.com/en-us/library/ms174378.aspx
http://blog.sqlauthority.com/2007/02/15/sql-server-fix-error-15023-user-already-exists-in-current-database/