I thought this was interesting:
The main idea is that some printers (or maybe most of them) have sensors that claim the ink is empty when there really is a lot left. Some people have found ways to fool the sensors, keep using supposedly "empty" ink cartridges, and still print a thousand more good quality pages.
It benefits the printer ink industry to have printers that jump the gun and signal for more ink too early.
Some printers can burn out if you fool them into using a near-empty cartridge, but others work fine even when ink is so low that the text being printed is barely visible.
Methods of fooling the sensor: 1) Cover the sensor in dark tape. 2) Color part of your ink cartridge with a dark marker. More info at the link above.