I had this ecommerce site selling widgets from manufacturer X. I built another site selling widgets from manufacturer Y. It was new and traffic was slow so I put in a square of adsense in the blank spots just for fun.
The first ad that appeared was my own adword for my own site, manufacturer X.
At first I thought this is bad. I'm paying for a customer I could have gotten for free with a link. Then I thought no this is good. I have another shot at the same customer and it is only costing me half price. Which is it?