A Google AdSense Help thread has one AdSense publisher complaining that there was "a $3,000 deduction" from his Google AdSense account.
Truth is, his actual earnings versus his estimated earnings was $3,000 less. Why? Because 3,000 of those dollars were actually invalid clicks and thus not earnings at all.
A Google AdSense Engineer named Guillaume said:
I confirm that AdSense receives a large proportion of invalid traffic from your site. This explains the discrepancy between estimated and finalized earnings that you are observing. I didn't find any error on your account; the existence of invalid traffic is corroborated by several signals.
Please note that you didn't "lose" $3K though, because invalid traffic does not generate income in the first place. What happened is that earnings were initially over-estimated due to the amount of invalid traffic received by AdSense on the site.
Of course, the AdSense publisher does not agree his traffic lead to invalid clicks. In fact, he has some solid evidence the traffic increase and ad click increases were legit. But that is a different story.
Lesson learned: Estimated earnings are not actual earnings.
Forum discussion at Google AdSense Help.