There are some sites that display not ads but fake virus alerts. When I visited one of these sites, I got a message blaring out of my computer to call a phone number or to reinstall my computer to get rid of a "virus." (Hopefully it was not.) Every time I closed out of the message box a new one would appear, and I finally managed to close out of the site. It's happened a few times. This may not entirely relate to the topic you were speaking about. :)