Out of curiosity how does the printer measure ink levels?
It doesn't. The carts have a chip that comes set as full. As the printer goes on printing it makes a 'guesstimate' of ink usage and records that on the cart's chip. That is what is displayed. The printers have no idea of the actual ink level and the 8400 is specially bad on this regard. Some nearly empty carts display 40% and go from that to empty.
As the 8400 have sub tanks, when you see the "!" on the ink level it actually means that the cart is dry and all the ink left is already on the sub tank, so you can replace the cart.
That said, I never saw an error message due to an empty ink cart, it simply says the ink ran out and asks for a new cart. There may be something else wrong with your printer, keep an eye on it.