Most AdWords advertisers would probably be surprised to see that they're only being charged for 75% or less of the clicks on their ads already. They simply never show up in the performance reports or on the bill.

To see them, log in to AdWords, click on a campaign, click the "Dimensions" tab, choose the "View: Day" filter, click "Customize Columns", choose the "Performance" category, and add the "Invalid Clicks" column to the report.

Now you'll see the huge number of clicks Google received on your ads but never charged you for.

It's also common to see adjustment entries on the billing report labeled "click quality". Those are automatic refunds for potentially fraudulent clicks given after-the-fact when they couldn't be caught in real-time.