From: http://docs.mongodb.org/manual/tutorial/troubleshoot-replica-sets/#socket-exceptions-when-rebooting-more-than-one-secondary

The following doesn't read well (emphasis mine):

EXAMPLE
Given a three-member replica set where every member has one vote, the set can elect a primary only as long as two members can connect to each other. If two you reboot the two secondaries once, the primary steps down and becomes a secondary. Until the at least one secondary becomes available, the set has no primary and cannot elect a new primary.