In printing, a hickey (also known as a bull’s eye or fish eye) is a spot or imperfection on a printed paper caused by dirt.
(above) When newspapers were “pasted up” by hand it was common for extraneous strips of paper with text on them, or nothing on them, to get dropped onto what would become the printing plate—and their images would be printed. (Newspaper article above was written by yours truly for the Brown & White at Lehigh University in 1966 -- when college students used slide rules, cigarettes cost 27 cents a pack and there were no iPads. However, sex had been invented.)
(above) It’s unlikely that you will encounter those problems in a book made with word-processing software, a PDF and print on demand or e-publishing -- but there is a 21st century version of the hickey.
If you use the Print Screen function of your computer, or software such as Snagit, you might accidentally capture an image with a cursor or pointer in it. Be careful.
top photo from Janek B. Thanks.