No-Prize
A No-Prize is traditionally a fan's reward for getting a letter printed in a Marvel comic that points out a mistake and comes up with a clever excuse for the mistake being there. Stan Lee would print the letter and tell the writer they had won a No-Prize – which was nothing.
Readers wrote in asking where their No-Prize was. Marvel began sending empty envelopes that explained that the No-Prize was inside.
Later, the No-Prize policy was changed so that the only way one could get a No-Prize was through the Stan's Soapbox column appearing in many Marvel comics. Stan Lee awarded them to anyone whose letter he prints in the column, or to those who solved occasional research challenges.