I've got ceilings that need to be painted, and I don't have any ceiling paint but I have a bunch of leftover white paint from painting my walls. Is there any reason I shouldn't just use my leftover wall paint for my ceilings? The wall paint has eggshell finish and the color is actually slightly off-white (white with a hint of cream)
Ceilings are typically painted with a flat sheen paint, as it's good at hiding (or at least not highlighting) imperfections, nor does it need to be particularly durable. Those two qualities tend to lead to a less expensive paint, and if you can save money where you can, why not. If you've already got some leftover (non-glossy) paint, there's no real reason not to use it. I wouldn't use gloss or semi-gloss on the ceiling, but eggshell should be OK.