This is a pretty minor usability issue. This is the result of running replSetSyncFrom on a primary:

foo:PRIMARY> rs.syncFrom("behackett-dt:27019")
{ "syncFromRequested" : "behackett-dt:27019", "ok" : 1 }
foo:PRIMARY> 
foo:PRIMARY> db.runCommand({replSetSyncFrom: 'behackett-dt:27019'})
{ "syncFromRequested" : "behackett-dt:27019", "ok" : 1 }
foo:PRIMARY> 

There is no log message that I could see (even at log level 5).

This should really raise an exception or at least warn the user that what they are doing doesn't make any sense.