Similar thing happened to me a while back. Flight from OAK to somewhere I don't recall. We boarded. Purser realizes he is one short. Some mumbling between purser, GA and FAs. They decide it is illegal for us to be on plane without requisite number of FAs. We all get shuttled off. UA tries to get standby FA from SFO. Bay area traffic strikes. We finally get out 50 minutes after their estimated (already delayed) departure.