the problem is, you can never say what click is invalid – e.g. double click on an ad – is it invalid? What if I closed the page, then forget what was its address, so i clicked again on the ad (it happens to me not so rarely) And the same for for many types of click fraud. BTW click fraud of my ads reported by google is now circa 10% of all ads. And during last 2 months i have measured the high drop of pages per visitor ratio on visitors comming from google (from 4 to 2), what is for me an alarming note, that click fraud on that ads are much bigger (around 30 %) just google do not resolve new methods of click "thieves".

On a site I monitored this spring (things change, I know) I was seeing around 30-40% of the clicks as being fairly certainly invalid, things like people clicking on the ads 20x in a row (not just refreshing), massive amounts of clicks from scraper and junk sites, clicks from pages that had no visible Adsense when I checked, etc. Google's total click count was perhaps 5% lower than the total count I saw. Are those 5% the invalid clicks which Google confirmed?

Yes, it's a PITA to use the content network. But on the other hand, it's just a part of the cost of doing business online. Maybe I was paying some 50-cent click-kiddie instead of paying for potential customers – as long as the bottom line is still fine it doesn't really matter to me. The last expo we went to, all sorts people were grabbing our pens like crazy, not just potential customers. Part of the cost of doing business :-)

Some browsers can be configured to prefetch links.
This may be common in browsers that Google sponsors (e.g. Firefox).

Does Google charge for the prefetch if it is never actually clicked?
Is this completely detectable by Google?

Philipp,
George R. asks a question that is of interest to me as well. I would also state for the record, and I have it on good authority, that you are right 84% of the time. That is 13% above the norm. Your awesome. ;o)

I see a lot of prefetched hits on my sites but I have never seen one come through Adsense / Adwords. Remember that Adsense is in javascript generated iframes ... I'm not sure any browser would prefetch that (it would be funny if the GWA did though :-)).