30 million dollars, that's how much director George Romero and his production team could have earned in 1968 for his box-office hit The Night of the Living Dead.

A silly editing error in the editing process put this cult zombie film into the public domain. And copyright-free means free, and no revenue!

How did this costly faux pas happen? In an attempt to change the title in the credits, Romero inadvertently removed the copyright notice, a mandatory prerequisite for the film to be protected by US copyright law.

The fatal change was discovered too late to reverse.

Filmmakers can now create with peace of mind: such a blunder is no longer possible since 1976, when the US legislature removed this requirement from the law.

American copyright law now applies a basic principle of copyright: a work is protected from the moment of its creation and without further formalities.

An instant classic, The Night of the Living Dead would have enjoyed massive distribution even if it had been copyrighted. But its automatic inclusion in the public domain was a real boon to TV stations and cinemas looking to fill their time slots cheaply.

But it was in another respect that this mistake had unexpected impacts.

The creatures in the film did revolutionise the archetype of the undead and contaminate popular culture. The lack of copyright helped popularise this cult film, but more importantly it created a new sub-genre in cinema: the zombie film.

Romero redefined the term "zombie" by creating a new type of monster that reflected the anxieties and plagues of modern society.

Before The Night of the Living Dead, several zombies had appeared on the big screen, but their nature was quite different. Far from the contagious, rotting corpse in search of human flesh, the pre-1968 revenant took the form of a dead (or sometimes even living) person controlled by a voodoo witch doctor.

Since the characteristics of the modern undead were born of George Romero's creative genius, their reuse could have put creators at risk of copyright infringement.

The lack of copyright has undoubtedly contributed to the surge of zombie hordes in the collective imagination.

George Romero may have been stunned by this legal blunder, yet he did not come out of it zombified. Romero and his colleague John Russo were the first to take advantage of this unusual situation: they each made their own series of 5 films!

And if it hadn't been for this technical error, would we have had Michael Jackson's Thriller music video? The Resident Evil series of video games and films? The hit comic books and TV series The Walking Dead?

Maybe yes, maybe not.

Could it be that the director's vision and creative genius are at the root of this monster phenomenon? After all, Tolkien redefined the literary fantasy genre, although his epic novels are still protected by copyright.

Pay particular attention to the black and white horror films that appear on the small screens alongside scenes from contemporary films and TV series.

You may recognise a Romero zombie.

Is it a nod from the director to their favourite film or simply because the images are free?