I am sometimes a little negative about CPC advertising because I usually find it overpriced. On the other hand, Vitabase does participate on a limited set of keywords that work for us.
Recently, due to slow sales, we discontinued a certain product. Unfortunately, we missed deleting the associated Yahoo CPC campaign because so little money was being spent on it. Then the product got some very prominant press coverage that revived it dramatically. As a result, we had a huge spike in expensive traffic to a page that no longer existed.
Because of the amount of money involved, I personally called Yahoo! Search Marketing and explained the situation. I told them that we knew it was our fault, but asked if they would work with us. At first, they offered only a token credit, but then agreed to research it further.