They were a staple in the letters columns in the back of comic books. A “No Prize” would be awarded when someone spotted an error, but came up with an explanation about how it really wasn’t a mistake because Marvel’s artists never made mistakes :rolleyes:
(an example I remember is a character had the sleeve torn off her uniform in a fight, a couple pages later she appears with sleeve intact, and next time you see her, no sleeve. The “No Prize” winning explanation was along the lines of she taped it back up or used pins that fell out easily)