Sometimes, the unprimer URL login may fail, e.g. bad login. However, since the only error indication may be a "Login failure"-type message and not a true HTTP error code, the Search Appliance may not be able to detect this and might continue walking useless (permission-denied or "Please log in first") pages.
To help detect such a unprimer URL failure, a Bad Login MM Query may be entered. If non-empty, this is a Metamorph query to run against the HTML returned from the associated unprimer URL. If it matches, the unprimer URL is considered a failure, and the walk is stopped for that particular site (other Base URLs will continue).