Inspector_Gadget - 5:10 pm on Aug 7, 2013 (gmt 0)
I've accidentally clicked on my ad probably half a dozen times in the last year.
As others have stated, Google is about catching fraud not penalizing people who accidentally click an ad. They look for patterns, trends and the frequency of said clicks. Google always filters out self clicks but if you are purposely trying to inflate your earnings, they will no doubt know this and ban you.
My AdSense rep even told me that invalid clicks in the simplistic term is okay - this happens and Google subtracts these in real time and at month end. But if it's patterned in a way to deceive the system or artificially inflate clicks, that is where you will run into big problems.