I went in the nether portal (as 2nd player, not host) from the nether. The building terrain screen popped up, but it did not load for a few seconds. On host's screen i was back in overworld, but it was still on building terrain. It finally teleported me (note i had full heath and full iron armor) and I instantly died of fall damage. I belive it happens when the host is in the nether or overworld while 2nd player joins, though thats not what happened the first time. The second time, I joined the game while host was in the nether, went to the nether portal, entered, and it did not load for 4-6 seconds, and when it did, I took 2 hearts of damage at a time until I died. Full iron armor, enchanted helmet (protection 1), full health.