What, you just expect Google to show you a giant red box saying "Whoops, we detected that you were trying to do an ad fraud! Better luck next time!". Google will happily show you whatever ads you request, but they won't count them as monetizable impressions and they won't charge advertisers for them. The goal is to make it impossible to tell as a bot operator whether your fraud was successful. So why give bot operators more info about how to evade your detections?