Anyone who every laid out a newspaper the old fashioned way (with wax and with lewd comments written all over the galleys in photo blue ink) knows the great April Fools fun that comes from swapping out the photo-blue mark-up pens with not-quite-photo-blue pens.
My mom used to write and sell ads for a family-owned newspaper. She would bring home galleys (laid out the old-fashioned way) to proof. We kids had a game called Find the Poop. Sometimes the typesetters would use their smallest font to insert the word ‘poop’ somewhere in her ads. Such small print would be illegible on the final, printed paper, so many an ad ran with poop on it.