MySQL reserves one connection to allow a super user to administrate the server even if it ever hits max connections:http://dev.mysql.com/doc/refman/5.5/en/too-many-connections.html

It would be very helpful if Mongo did this as well.

I've run into the problem of too many connections a few times now; most recently, when I was mongorestoring a database and the indexes were being created (presumably in the foreground?), the server filled up on connections and I couldn't do anything but watch the server spit out "Too many connections" error messages. I couldn't step it down, and terminating the mongos processes didn't help. In the end I was forced to just kill -9 it in order to fail over to the secondary.