Mongos currently assumes that if it can't find the config.version document and the config.shards is not empty, then the config server metadata is pre v2.4. The problem is that mongos queries the config.version first, and the config.shards later. Which means that another mongos could have inserted the config.version document and populated the config.shards in between the config.version and config.shards query (or alternatively, the secondary would have catched up to that point).