I have been testing a mob farm in the Nether that I originally built in 1.5.
After making a few changes I was going to test it again and to my surprise it got filled with mobs from the Overworld, zombies, endermen, witches and even chickens! Also, I have not seen any ghasts for a while...
I tried it both in creative and survival.
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (
[minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]
) here.