Google AdSense Adjustments

Another round of monday questions:

Hi Jeremy, Recently I've been slapped with a click adjustment from Adsense for invalid clicks. As I've spent literally thousands of my own money to advertise my sites on other PPC networks, I'm worrying myself sick over the possible outcome of the adjustment. Will I end up oweing them money??? I've done everything aboveboard, complying strictly to their TOS and putting my own money into the business. As of writing this email, Adsense is still calculating the amount to be deducted from my account.

I have heard of this before but I have never seen anyone have to owe AdSense money. Maybe if anyone else has ran into this first hand they might be able to comment?