Google has published a report on invalid click data, and have outlined the measures taken to protect advertisers:

“For example, an advertiser with a $10 daily budget could be attacked by someone attempting click fraud consisting of $1 million worth of clicks. When our filters protect against that attack, the advertiser’s invalid clicks rate would increase dramatically – meaning we were filtering out a very high proportion of their clicks – and their campaign would be unharmed. Similarly, that attack alone would increase the overall invalid clicks rate on our network, even though it was limited to a single advertiser. In this manner, a large attack focused on just a few advertisers can actually manipulate Google’s overall invalid clicks rate for that day, so this is an externally manipulable number“