A blockchain fork? It's not like one miner in a pool can be off mining down the wrong blockchain since work is delegated by the pool server.

At least, that'd normally be the case. Biblehash is a little unique and involves a reference to the previous block in the chain which requires miners to run their own full nodes, so I suppose it might be possible that a user in the pool would be attempting to mine while referencing an orphan block, which would probably result in all their shares being rejected. I don't know if the pool checks to make sure all users are synced properly. I don't think that's the case here though since all the hashes from the logs seem to match up with the test blockchain.