Point of clarification...
AdSense is for publishers to make money by displaying ads on your site.
AdWords is for advertisers to get targeted traffic to your site.
PPC advertisers in general must assume that some fractional part of the clicks that they pay for will be essentially for naught. Could be an accidental click, could be a fraudulent click, maybe even your site might be down at the moment. I think that Google trys very hard to not make advertisers pay for fraudulent clicks, so I don't that it's too bad, but use a smaller PPC network on a competitive term then watch out...