Google said my sites have invalid clicks and my AdSense account got banned. I got Google's email in Feb which also said my sites had innvalid clicks. To protect myself, I purchased third party AdSense stats script and threw several IPs to my sites' blacklist.
But my account was still disabled. WTF!
It seems if you have competitor, and if he has AdSense on pages, You may choose to click his Google Ads again and again. I was hit. Hopefully you will not.