When using GSSAPI against a Windows mongos, the error message for user not found is unhelpful. Note that the user attempting to login must be a valid user in the KDC, just not one that has been added to MongoDB.

2013-11-08T07:47:04.549-0600 Error: 8 invalid parameter: expected an object () at src/mongo/shell/db.js:1199
exception: login failed

The same error message against a mongod looks like this:

2013-11-08T07:47:26.222-0600 Error: 11 User not found at src/mongo/shell/db.js:1199
exception: login failed