A DigitalPoint Forums thread reports that Google sent an AdSense publisher an email that $1,233.46 has been deducted from his account due to invalid clicks. The publisher posted the email he received from Google:
Thank you for your email.
As you know, we recently completed an investigation into invalid click activity that we detected on your account. Based on the findings of our investigation, we've deducted $1,233.46 from your account earnings. This amount represents the earnings previously credited to your account from invalid clicks, and the deduction of these earnings will be reflected in your next scheduled payment.
In addition, because the amount of this deduction may exceed your April earnings, we've placed your account on payment hold for this pay period. Any outstanding earnings will roll over to the following month, so you'll be paid out in accordance with our normal payment cycle.
Lastly, I understand that you may want more information about the invalid activity we found on your account. However, because we have a need to protect our detection systems, we're unable to provide our publishers with any details about their account activity. We appreciate your understanding.
I have no proof of the authenticity of this email, but I would guess it is real.
In any event, it appears this publisher was aware that this may happen. In fact, he calls himself lucky for not being banned. He said, "I am lucky I am not banned" but he blames the invalid clicks on "proxies" who have tried to "sabotage" his efforts as a publisher.
Forum discussion at DigitalPoint Forums.