printer problems

Why do printers always run out of ink in between jobs? Seriously, I had a color print job. I test printed, and it looked fine. I hit print for ten copies. I came back, and the black had run out - on the first copy. Then, on my other crappy, b/w printer, I test printed one, and it looked fine, inkwise. Then I did another, and it came out blank. How could it have gone from inky to bone-dry in between letters? If the printer has some sort of circuit to control this, why does it print at all?