Once we run replSetInitiate, the primary has to perform a quorum check and communicate with the secondary. In a mixed replica set, if the primary is started with the 5.1 binary while the other is started with 5.0, the primary sets its FCV to 5.1, then to "downgrading to 5.0", and then finally to 5.0.

While the primary is on 5.1, its wire version will be incompatible with the secondary's. We are having a race, which causes replSetInitiate to fail, since the wire versions are incompatible.

This failure may extend to isself_failure_restart.js and isself_failure_reconfig.js too.

Please see comments.