Google Adsense sometimes displays a Public Service Ad (PSA) on the pages monetized through Google Adsense. These PSAs are non paying ads and are meant for charity. Google defines Public Service Ad as

PSAs are non-profit organization ads that are served to pages when targeted ads are unavailable, or when Google is unable to gather content from the page. Publishers do not receive earnings for clicks made on PSAs.
Some of the main reasons for PSA being displayed are:
1) The page is not crwaled by google or in same way has restricted access (like through robots.txt)
2) No relevant ads are available for content related to the page.
3) Modification in ad code.
You can read the complete list of reasons here.

Today, I read a post by Amit Agarwal on Digital Inspiration, which questioned about the recently served.
Even I was feeling bit fishy about the same. I have seen quite a number of pages being served with that election 2008 PSA. Although I strongly feel that there should be some relevant ad available for the content of those pages.

Also that election 2008 page is certainly not non commercial. Still Google displayed the ad as PSA and publishers were not paid for it.

And now guess what. After all this questions raised, the election 2008 page on google server is deleted.
Check it out here
http://www.google.com/2008election/

Screenshot: (click to enlarge)

The page existed well as it has pagerank 6.
Also here is the screenshot of page from google cache. (click to enlarge)

I usually feel bad while questioning Google, but someone needs to answer this. They owe an answer to all adsense publishers.