Visitor Thomas Dirks alerts us to the OpenNet Initiative‘s study on the company’s July 25-28 blockade of the Voices for Change web site.

According to the study, Telus also blocked at least 766 additional web sites in the process.

Voices for Change’s host uses the common practice of sharing an IP address — the unique number assigned to a computer connected to the Internet — among many sites. So when Telus blocked Voices for Change, they also prevented their more-than-a-million customers from seeing every other site sharing that IP address, including a fundraising web site for breast cancer.

If there’s any good to come out of this colossal screw-up, the OpenNet folks have found it:

As this case demonstrates, seemingly compartmentalized decisions to block access to Internet content can have drastic unintended consequences, barring Internet users from reaching hundreds of unassociated Web sites. ISPs can unilaterally block access to large swaths of content with the flick of a switch. By collaterally blocking hundreds of completely unrelated sites, Telus has vividly demonstrated the dangers of Internet filtering, particularly when it is conducted in an arbitrary and unaccountable manner.