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A WebmasterWorld thread reports how someone with an Invision Power Board, a popular discussion forum software, found Google AdSense code embedded on his site, without him doing it himself.
So if you have forum software, double check that it is your AdSense publisher ID and code in the site and not someone elses.
If you see something sketchy, you should report it to Google at AdSense Abuse. More details on reporting violations at Google AdSense Help.
Forum discussion at WebmasterWorld.
