everytime you inadvertently hit the ads at the right hand side of the page you are on...it costs the advertiser
most of the time, I hit those ads in error I did not realize it was costing someone money it seems grossly unfair I have often complained about the ads being on the rhs, like right next to the scroll bar if they were honest, they would have the ads above or on the left (majority of people are right handed)
I was just looking at internet advertising rates, and they count as page impressions....so I would not be using the sidebar for advertising