PlayerInventory.getHelItemSlot() doesn't returns the real slot si it makes the method useless when you want to compare with InventoryClickEvent.getRawSlot() for example.

In fact PlayerInventory.getHelItemSlot() returns the slot beginning from the quickbar and not from the off hand slot. It will return 0 if the held item is the first of the quickbar, but this slot is actually the 36th.