I was in my server with my friends and we were fighting the ender dragon. After we killed it and jumped into the end portal, only 1 person could jump in and when the rest did the game would break. You couldn't chat, open your inventory, press esc, anything, you just had to close the game somehow and restart. This has happened about 3 times now and it doesn't need to be when fighting the dragon. It happens when more than 1 person tries to go into the end portal around the same time.