Calling db.auth() with a document as the arg mutates the document by adding a "mechanism" and a "userSource" or "db" field to the doc. This prevents the same doc from being used for a future auth call.

> var authDoc = {user:'spencer', pwd:'pwd'}
> db.auth(authDoc)
1
> authDoc
{
        "user": "spencer",
        "pwd": "pwd",
        "mechanism": "MONGODB-CR",
        "db": "test"
}