Action Comics #1, containing the first appearances of several superheroes
A blurb in the "Odds 'n Ends" section on the inside back cover of Action Comics #1, from 1938, mentions that Grissom is anxious to pitch both games of a doubleheader one day.
The first panel of a comic featuring I.H.O.T.F.M-Man is usually a parody of the cover of Action Comics 1.
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In the first episode of the fifth season of The Simpsons, "Homer's Barbershop Quartet", Homer Simpson, along with finding an original copy of the U.S. Declaration of Independence, an Action Comics 1, and a Stradivarius violin, all of which he throws away, then comes across a full sheet of Inverted Jennys in the 5 cent box at a local swap meet.
An example is Action Comics #1, the first published appearance of Superman.