Last night, my mongos died with this message.
Tue Nov 9 03:34:22 [conn5875028] uncaught exception (remotes don't match old [10.4.1.7:51675] new [10.4.1.8:51675])(mongo::UserException) in PortMessageServer::threadRun, closing connection
Those IPs are to two servers that do heavy writes to the system. I noticed it first thing this morning and restarted mongos. It came back online without any problem.