Hi, i just had experienced the same problem. This issue usually comes from using a bukkit world in a vanilla server.
See this bug report:
https://mojang.atlassian.net/browse/MC-4560
Bukkit sets the NBT value of "CanPickUpLoot" to 1. This value triggers some strange behaviour in a vanilla server:
Whenever you pick up an item you don't have already in your inventory it will be "equipped" into your boots slot.
When you pick up boots they will go into the trousers slot.
When you pick up trousers they will go into the chest plate slot.
When you pickup a chest plate it will go into your helmet slot.
When you pick up a helmet the game crashes because it tries to equip it in the next slot which doesn't exist. That's why an ArrayIndexOutOfBoundsException is thrown.
Since a pumpkin can be used as helment it crashes the game when you pick it up.
Just wanted to clear this up. It is actually a bug in the vanilla server which is caused by worlds created / played with bukkit and has nothing to do with Forge. I fixed my issue by using NBTExplorer and changed CanPickUpLoot back to 0
