Str82u - 3:49 am on Dec 29, 2012 (gmt 0)

@Marked - this happened to one of our domains and it was a third party browser toolbar that was cauing it and I believe Opera had a database of it's own that it accessed before the user hit the page.

If you check the settings for IE there's one that does just what you're descibing; it checks the page to be visited first against a dbase of sites then if it checks out to be clean the browser opens the page (I think it is on by default but can be disabled). It's been a while but I believe we had to submit requests to whoever maintains that dbase or another just like it - it was happening to us here as well as users.