What is overprint in printing?

By default, printing software knocks out: place yellow type on a blue background and the software removes the blue where the type sits, so the yellow prints on bare paper and looks yellow. With overprint on, the blue prints and the yellow prints on top of it, and the two inks combine into a muddy green.

That behaviour has legitimate uses. Black text is very often set to overprint deliberately, because black is dense enough to cover whatever is beneath it and overprinting avoids any registration problem around the letterforms entirely. Deliberate colour mixing and certain trapping effects also rely on it.

The failure mode is spectacular and quiet. A white element accidentally set to overprint disappears completely, because white in CMYK means "no ink" — printed on top of a colour, nothing happens and the element is simply not there. It looks perfectly fine on screen in most design applications, which is what makes it dangerous.

Preflight checks for it, and proofing from the ripped file shows what will actually print rather than what the design application chose to display. If you take one thing from this entry: never leave white or light elements set to overprint, and check the setting rather than assuming the default.