If login fails, Piwik (1.10.1) might give the user a link to click. If the link is clicked the following nonsensical error message is shown:
Invalid Referer detected - check that your browser sends the Referer header. The link you would have been redirected to is: http://piwik.org/faq/how-to-install/#faq_98
In 2f9f61511f071e3b8ec706595660468e22fd5d9a: Fixes #3751 did you mean the message was not clear? hopefully this one is slightly better. But the error is accurate: you are not sending the referer header and this is required for security reasons.