In a replica set, if a resync operation is attempted on a node before it loads a valid replica set config, the mongod process crashes.
A newly started mongod with the --replSet parameter does not immediately have a config; it must first load a valid config from disk, have a config delivered to it from another node, or have the replica set initiate command run by an admin.
The mongod process crashes, and a stack trace is printed in the log. This only affects newly started mongod processes that have not yet had a chance to join a replica set, so the impact of this issue on a replica set is minimal.
Do not run resync on a mongod before loading a valid replica set config.
MongoDB production releases from version 2.6.0 up to 2.6.3 are affected by this issue.
The fix is included in the 2.6.4 production release.
Do not allow resync commands if the replica set config has not yet been loaded.