Program: 22437
Project: 22437
Buggy source:
/** 
 * Called when a player shift-clicks on a slot. You must override this or you will crash when someone does that.
 */
public ItemStack transferStackInSlot(EntityPlayer par1EntityPlayer,int par2){
  ItemStack itemstack=null;
  Slot slot=(Slot)this.inventorySlots.get(par2);
  if (slot != null && slot.getHasStack()) {
    ItemStack itemstack1=slot.getStack();
    itemstack=itemstack1.copy();
    if (par2 == 2) {
      return null;
    }
 else     if (par2 != 1 && par2 != 0) {
      if (itemstack1.getItem() == Items.water_bucket || (itemstack1.getItem() instanceof IFluidContainerItem && ((IFluidContainerItem)itemstack1.getItem()).getFluid(itemstack1).getFluid() == FluidRegistry.WATER)) {
        if (!this.mergeItemStack(itemstack1,0,1,false)) {
          return null;
        }
      }
 else       if (TileEntityFurnace.isItemFuel(itemstack1)) {
        if (!this.mergeItemStack(itemstack1,1,2,false)) {
          return null;
        }
      }
 else       if (par2 >= 2 && par2 < 30) {
        if (!this.mergeItemStack(itemstack1,30,38,false)) {
          return null;
        }
      }
 else       if (par2 >= 30 && par2 < 39 && !this.mergeItemStack(itemstack1,3,30,false)) {
        return null;
      }
    }
 else     if (!this.mergeItemStack(itemstack1,3,38,false)) {
      return null;
    }
    if (itemstack1.stackSize == 0) {
      slot.putStack(null);
    }
 else {
      slot.onSlotChanged();
    }
    if (itemstack1.stackSize == itemstack.stackSize) {
      return null;
    }
    slot.onPickupFromSlot(par1EntityPlayer,itemstack1);
  }
  return itemstack;
}

Fixed version (ground truth):
/** 
 * Called when a player shift-clicks on a slot. You must override this or you will crash when someone does that.
 */
@Override public ItemStack transferStackInSlot(EntityPlayer par1EntityPlayer,int par2){
  ItemStack itemstack=null;
  Slot slot=(Slot)this.inventorySlots.get(par2);
  if (slot != null && slot.getHasStack()) {
    ItemStack itemstack1=slot.getStack();
    itemstack=itemstack1.copy();
    if (par2 == 2) {
      return null;
    }
 else     if (par2 != 1 && par2 != 0) {
      if (itemstack1.getItem() == Items.water_bucket || (itemstack1.getItem() instanceof IFluidContainerItem && ((IFluidContainerItem)itemstack1.getItem()).getFluid(itemstack1).getFluid() == FluidRegistry.WATER)) {
        if (!this.mergeItemStack(itemstack1,0,1,false)) {
          return null;
        }
      }
 else       if (TileEntityFurnace.isItemFuel(itemstack1)) {
        if (!this.mergeItemStack(itemstack1,1,2,false)) {
          return null;
        }
      }
 else       if (par2 >= 2 && par2 < 30) {
        if (!this.mergeItemStack(itemstack1,30,38,false)) {
          return null;
        }
      }
 else       if (par2 >= 30 && par2 < 39 && !this.mergeItemStack(itemstack1,3,30,false)) {
        return null;
      }
    }
 else     if (!this.mergeItemStack(itemstack1,3,38,false)) {
      return null;
    }
    if (itemstack1.stackSize == 0) {
      slot.putStack(null);
    }
 else {
      slot.onSlotChanged();
    }
    if (itemstack1.stackSize == itemstack.stackSize) {
      return null;
    }
    slot.onPickupFromSlot(par1EntityPlayer,itemstack1);
  }
  return itemstack;
}