If you are using [tag]PPC[/tag] for your [tag]marketing[/tag], then you should start to worry about click [tag]fraud[/tag]. This is a problem that seems to not go away. Your competition could be clicking away on your ads just to make you pay. I am not sure how you can actually know if somebody is doing this, other than you being suspicious of something not right is happening.
According to Coracle Inc, they discovered an episode of click fraud this week. Read the story here.