I was jumping between portals to go to my monster spawner under ground. I was in the nether, then entered the portal. After that, it wouldn't return me to the over world. I was stuck on the loading screen for about 5 minutes before closing the game and restarting it. After restarting it, I also cannot join my friends world again. They have to restart their world in order for me to join. This bug makes the game unplayable in some cases.