If they believe YOU the webmaster is behind the invalid clicks they will take action - such as buying traffic from known shady dealers etc. If they see it's just a lost bot, an idiot trying to get you turfed etc they will usually just roll back the clicks/rev without saying anything.
I saw not long ago in my own account - 2 impressions - 50 clicks on one unit. By the next day it was reversed.
I also have a client that was clicking his own ads to try to find competitors so he could edit them out through the adsense control panel - anyways they just deducted the earnings but left all the clicks there - about two dozen of them. After three days they finally sent him an email telling him about the preview tool.
