The Internet Watch Foundation (IWF - www.iwf.org.uk ) was set up as a kind of internet police officer, a place where people could go to report illegal content they found online. Its main focus is child pornography.
It acts on tip-offs sent in by members of the public, but one recent tip caused uproar last week. It concerned a page on Wikipedia.com, about '70s rock band Scorpions. Their album Virgin Killer featured a distasteful picture of a naked girl, and it was this that caused the report to IWF.
The normal IWF process swung into action. The page was added to the IWF's blacklist - but thanks to technical details that govern how the blacklist works, there was an unintended side effect.
Suddenly, thousands of UK internet users couldn't edit any pages on Wikipedia at all. Nor could they open new user accounts.
Wikipedia is supposed to be the online encyclopaedia that anyone can edit, but now many regular editors were blocked. They were furious.
Many news outlets reported the issue but concentrated on the Scorpions album cover image. The real cause of the the uproar was the IWF's handling of the case. As an album cover, the image remained visible on many thousands of music sites, which were not subject to IWF blocking.
Critics said the IWF's processes were at fault; others said it was all a technical problem. What was news to many people was that the IWF - an unelected and largely unaccountable body - operates a blacklist of websites which most of the UK's largest ISPs automatically subscribe to.
After days of discussion and debate, the block was removed and the IWF said in a statement: "IWF's overriding objective is to minimise the availability of indecent images of children on the internet, however, on this occasion our efforts have had the opposite effect. We regret the unintended consequences for Wikipedia and its users."