Click fraud is a major issue with all PPC engines, but to make it even worse, there are people out there using click fraud to hurt people out of spite. There is a thread at WebmasterWorld named Revenge by click fraud - what to do?, where a few members discuss the fact that they are aware of people who repeatedly click on their AdSense ads simply to raise a red flag at Google. The clip-happy people are looking to have their AdSense accounts banned and destroy their AdSense income.
In the thread, they discuss way to prevent click fraud. None of which are fool-proof.