Reason of Showing PSA in Google AdSense (

I've already had some experience with the way adsense works. Oh, and did I mention, I've discovered several AdSense NO-NOs. Oh and I also figured out when and why google displays public service ads. I figured that Google will usually display public service ads in one of the following scenarios:

1. Your publisher ID is wrong or improper. Or perhaps you did not comply to the google TOS.
   
2. You tried to modify the adsense code and accidently screw it up.
3. Google can't find any ads to match the content in the page.
4. You did not put the code in the template, and put it in a post instead - The thing that you should keep in mind is always have the adsense in your TEMPLATE ONLY. Don't try and put any adsense code into your posts, because it will not work. Instead you'll just get a ugly ad like the one shown below. Why? because once you post the code through most CMS systems the code will not stay in different lines (the linebrakes between the code will be removed), therefore it wont work.
5. Google might have "flagged" your page? (see below for more stuff on why I think that might be the case)
   
6. There was a hardware malfunction that occured at Google HQ.

Src = http://beyondteck.blogspot.com/2007/09/google-adsense-public-service-ad.html