Back to the little rubber things - on my 801, the cartridge won't stay inside the outer cartridge thingy. I don't know what you call it, but the larger white thing that the cart fits in. When I pull the cart out of the other thing, the little rubber thing stays inside. Unless I dig down inside with a paper clip and put it back on the cart, the two pieces won't snap back together. Which wouldn't matter all that much, but storing them to refill later is a mess if they aren't snapped together. I know that's a really bad description, but I don't know any other way to put it.
The 901s aren't nearly so annoying.