"Inflict a prank call"
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If you're talking about the one on top that says "Ads by Google", yes it is random. Although it will usually choose ads that are related to the subject of the website. For example, here on chess.com, it is: Chess. :)
You know us chess players are a bunch of pranksters. We're being targeted for our weaknesses.
I too have seen the advertisment and considered the inappropriatness of the subject matter, however, I am a "rock" where advertising is concerned. I am NEVER persuaded to buy, or even intrigued by advertising of any description. As far as I am concerned the site may place whatever adverts it sees fit, in the full knowledge that I will be unaffected by it. If it helps chess.com to remain solvent then "crack on!" I say. 
I like that, "crack on".
I've noticed an advert to "inflict a prank call" appearing on Chess.com which directs to www.makeprankcalls.co.uk. I understand that Chess.com has to start making money, but this hardly seems to be an appropriate advert for the website.
It's currently on this page: http://www.chess.com/photos/view_album/doingme/los-photos
I wonder why this advert was chosen, or is it some kind of google ad where you can't specify what it is?