This goes along with the topic that I just talked about, but there are a few more things to consider. If you have an ad that realizes a lower amount of revenue per click and outperforms your ads that have a higher revenue per click, your ads are said to be cannibalizing each other.
A classic example of this is with Google's Ad Links. Many people include Ad Link units on their page because they are so easy to “blend” into the content on your site. However, ad link units also typically have a lower revenue per click than regular ads.
If you have an adlink unit that is getting a significantly higher number of clicks - and your users leave your page for good, you may be losing money.
Cannibalization can also occur when you place other types of ads on your website. Do you have another approved contextual ad unit on your page that is not as profitable as Adsense? Is it taking clicks away from your adsense ads?
Not all competiting ads will cannibalize each other, which is why it is important to always test your results.
