I saw a site that had a few cheap flash games for kids. When you click on a game, it pops up into a different window infront of the original. The close button for this window is right over the first ad in the Google adsense column. Any 'misclicks' are going to result in a tidy little profit for the site. You can't tell me that that wasn't deliberate.
Oh well. I wish people would focus on the long term and build sites that will survive the ebb and flow a single ad network. But then again, if there is money to be made . . .
If they generate too high a CTR, they'll draw the attention of the adsense admins, who'll take a look at the site and see for themselves. If they're deemed to be breaking the T&C's, their contract will be terminated.
It is also technically a violation of the AdSense policies to have a pop-up like that, since the window covers (or partially covers) the ads:
Site may not have: Pop-ups that interfere with site navigation, obscure AdWords ads, change user preferences or prompt a user to change his/her preferences (such as a userís home page), or are for downloads
Slick trick, I suppose, but will get the publisher one of those dreaded emails ;)