In v2.4 a user defined on db A can be given privileges on db B by adding a "surrogate" user document on db B. To support this functionality in 2.6, whenever we do an authorization check for a v2.4 style user, we check if there is a surrogate user doc for that user on the database being accessed. We keep track of which databases we've already checked for surrogate documents for each user, in that user's list of "probed" databases. The problem is that if there is not a surrogate doc for a user on a given database, we neglect to mark that database as probed, which means that the next time that user does an operation on the same database it will try to check for a surrogate doc again, every single time. This can cause serious slowdowns in a system using a 2.6 mongos with v2.4 style users.