Starting in 2.8 mongod will fail to startup if it has an invalid replica set configuration stored in local.system.replset. We need a docs page describing what to do in this case. Before we can push the change to the server, however, we would like to at least have a URL that we can include in the log message when the server fails to start up so we can direct users to more information on how to recover than we can put into the log message.

The steps the user should take most likely depends on what the reason the config is invalid is. There were a few changes in 2.8 as to what makes a config valid/invalid, for example it is illegal starting in 2.8 to specify a "votes" field with values other than 0 or 1. So if the user has a config that was valid in 2.6 but is no longer valid in 2.8, then they should downgrade to 2.6, reconfigure to a future-proof config, then re-upgrade.

If their config is invalid in another way, then they must have done something funky to even get this config stored - their best bet is probably to restart without --replSet and on a different port and drop local.system.replset. Though I'm not sure whether we want to actually document that part as this case should be very rare, and we have to be careful about recommending people mess with system collections directly.