Back in 1968 when it was released, in order to claim copyright on a film, the copyright had to be stated on screen; As was usually the case, George A. Romero's movie 'Night of the Flesh Eaters' claimed the copyright on its title card. When the studio re-named it to the now familiar 'Night of the Living Dead', they didn't put the notice on the new title card.
The result of this is that the most famous Zombie movie of all time is in the public domain.