It's an issue with the ad banner - Muhammad needs to work this out. Whenever anything but the lawnmowerforum home page is loaded, it also attempts to load the following URL:
http://googleads.g.doubleclick.net/page
This returns an error 404 (page cannot be found), which is what you see at the top of the page. This issue also affects the browser "BACK" button - you have to click it twice to get "around" the bad URL for the ad banner.