I received a Warning E-Mail about a year ago for fraudulent clicks... Luckily my account is still alive and going!
I received a warning because of some auto-surf traffic. Some guy I was helping decided to "do me a favor."
I've received 2 warning emails in a row now..Some of my sites are not compilant with google policies so they asked me to taken off the adsense on those sites.
I received a warning for a site I had purchased which had been using traffic exchange networks. I quickly made sure that the old owner removed the site from the exchange networks and notified Google of my course of action. Today, this site still runs adsense.