I will have to dig up the news article, but there was a story run recently that The New York Times, CNN, Comcast and bunch of other big name sites were showing malware ads... The ads could install Crap without you even clicking on it.
There are tons of that kind of shenanigans going on all over the place. And now these major sites are doing it?! And I thought the ads targeted to teacher resource websites were bad enough. Luckily...adblock plus for FF blocks 99% of that crap.