A man using the British Library's wi-fi network was denied access to an online version of Shakespeare's Hamlet because the text contained "violent content".
Author Mark Forsyth was writing his book in the library, and needed to check a line from the famous play.
The British Library said the fault was caused by a newly installed wi-fi service from a third-party provider.
One security expert said the incident highlighted the "dysfunction" of internet filters.
Mr Forsyth revealed on his blog that the filter had logged his attempt to access the page.
A spokesperson for the British Library said Hamlet had since been made accessible.
"The upgraded service has a web filter to ensure that inappropriate content cannot be viewed on-site," he added.
"We've received feedback from a number of users about sites which were blocked, but shouldn't have been. We're in the process of tweaking the service to unblock these sites."
Internet filters have recently come under increased scrutiny, after the government announced that pornography will be automatically blocked by UK internet providers, unless customers choose otherwise.
Digital rights activists raised concerns about the move, fearing that the lists of "banned" sites could be expanded to include pages that should be publicly available.
Prof Ross Anderson, a security expert at Cambridge University, told the BBC that internet filters were "pointless" and that it was "completely inappropriate" to have one in the British Library.
He added: "Everything that is legal should be available over the library's wi-fi network. The only things they should block are the few dozen books against which there are court judgements in the UK.
"One of the functions of deposit libraries is to keep everything, including smut."
The British Library defended its position, saying that it wanted to protect children visiting the building from content "such as pornography and gambling websites".