They call themselves "Anonymous", and they are the world's most famous group of hacker-anarchists. When they have left the glow of their computers to protest in public – against anti-piracy laws, perhaps, or the imprisonment of Julian Assange – they have taken, very wisely, to wearing masks. Since 2008, the mask of choice has been the eerie "Guy Fawkes" design made famous by the film of Alan Moore's graphic novel V for Vendetta.

In Moore's story, the mask is worn by a lone freedom fighter against government iniquity. Yet it is a measure of the allure of hacktivism that the real-life replica has now become one of the most popular masks worldwide. Its manufacturer, Rubies Costume Company, sells well over 100,000 every year, and the product is the best-selling mask on amazon.com, amazon.co.uk and amazon.de. In the words of one reviewer on the site, it is "very useful to hide your identity from the public while you go about your anonymous deeds".

Now, it is not nice to sneer – nor is it wise, when one's target is a rather touchy criminal collective. But there is a tasty irony about the fact that the V mask is itself a copyrighted product. Every time that Rubies sells one – for $6.49, £5.16 or €10.50 – a cut of the profit goes to Warner Bros, which made the film. That's Warners as in one of Hollywood's six big studios, a subsidiary of TimeWarner, and a member of, yes, the Motion Picture Association of America – Anonymous's adversary in the fight over online piracy. It just goes to show. The battle for copyright may be lost, but no one flouts the law of unintended consequences.