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.

Comments:
Michael Martinez
04/10/2012 05:45 pm
At least they didn't cancel his account.
Adam
04/11/2012 04:51 am
was the money returned to the advertisers ??? I never saw back credits from adwords for charging on bad clicks just a thought
Guest
04/11/2012 06:05 am
I noticed that when earning increase then deduction is more respective to less earning. My traffic did not increase but I changed in ads position, color etc. and my earning increased so Google deducted more respective to previous months. Earlier, I was using social bookmarking and other tactic to increase traffic at that time there was chances of more invalid click and traffic but that time deduction was less because earning was less. When I stopped these all thing, traffic was same but I did little changes in ads, earning increased and deduction increased. I was really surprise. There is really big hole in Estimated earnings and actual earnings. I think, Google engineer should take it serious. They are not correct in all cases.
Damir Karaica
04/11/2012 08:16 am
:) If google say that's invalid clicks, that's invalid, and than we loos money Dadony
Angela
04/11/2012 08:58 am
Damir is absolutely right if google say that then nothing to do we lose money......
Vikrampal
04/11/2012 11:17 am
We all know google adsense is top earning source for all webmasters. But big problem with google adsense is that if something wrong with adsense then we can't do any thing its really big problem. Its very rare if someone get any help from adsense.
Yagnesh Darji
04/11/2012 01:42 pm
Well my earning not decrease single penny all as it is.. :)
divyansh
06/04/2012 05:38 pm
i lost my account for this same reason... hey i have a question...its off topic but do reply .. what css code are you using for your discuss comments...its great