All of the measured campaigns run only on the Google site and Search network. AdSense is a click-fraud-wonderland, in my opinion, so I avoid it...
Interestingly, there is a correlation between campaigns which drive CPA traffic and increased 'invalid clicks'. Several holiday campaigns that generated affiliate leads had 8-10% invalid clicks, according to the Google report.
Perhaps that's competitive clicking? I need to investigate more, but it's barely worth the time.
Want to know what Google thinks your invalid click rate was? Google doesn't tell you this by default. You can find it by going to Reports, running a Campaign Perfomance report, and choosing the Invalid Clicks field under Advanced Settings -> Add or Remove Columns.