As Wesman says it makes no difference if it happens the odd time. They won't pay you any money for it though. If they think it's deliberate, you are in trouble, however, as they will ban you!
I've accidentally clicked an adsense ad of my own about three times over the past 4 months, normally when the screen is slow to load and I'm trying to click on something else.
You are allowed to click other people's ads though. Provided you have a genuine interest in the advert (ie you're not in a mutual clicking agreement).