This is probably the worst bug it can happen to you, inthis bug when you login into on your account and enter the game the world you play on is simply gone it doesn't show you your world in the list of world it happened to me 2 times the first one i ignored it but the secont time it happened my favorite world which i was working on it for 1 year and there was it wasnt on the world list. I hope Mojang fixes it quicly because it can happen again and its so frustrating to see your beatifull world which you have been working a lot on it  disapear and lose all the progress and start over from scratch.