AdWords is one of the most complex project I have ever worked on. AdWords involves huge complex calculations and collections of data and stats. It has various salient features and has a very heavy duty traffic of zillions ..trillions end users. These zillions..trillions end users are expected to perform various combination of actions and transactions. Testing such an application is in itself a challenge. Above all, even thinking about automating such an application was definitely complex and challenging!!
Certain aspects to take into consideration for automated production testing: