Forum Moderators: phranque
1) Another website has posted an invalid URL to your site, and users are trying to click it. Mack is right: the %20 is a space symbol.
2) A page on your site has the invalid URL (http://%20www.example.com) and users are clicking on it (from within your site)
Check the "Visitor Path" statistic in StatCounter. That should tell you where the visitor who clicked the URL came from.