I always have issues with Adwords. Google overcharged my credit card, disapproved my ads, did other unfair things. But i guess the worst thing that can happen is that they will remove you from natural search results if your adwords bid is lower than what they think it should be.
Let's see... I have a site which is an alternative to u-sendit.com. It was on the first page in natural results in Google for a few months on the "email large files" search term.
After i signed up with Adwords and started bidding for "email large files", guess what happened? The link was gone from the first page completely. So when i had about 20-60 free visitors a day from Google before, now i still have the same 20-60 visitors, but they are not free anymore! Each damn click costs me $.50!