Adsense Hijacked - After doing a periodical check, I noticed an old domain of mine with low traffic has an adsense publisher code that isn't mine. I looked in my adsense logs and it seems he replaced my code with his sometime in February.(a few months ago) I can see a complete stop of page impression on some channels....
SarahTech35 forumer at webmasterworld.com

Volume Verification - Running Adsense Validator through our hosted topsite service identified immediately the users who were replacing our ad code. Google does not allow multiple publisher ID's on the same page, so this helps keep our service in good standing with google.
Mark at itopsites.com