The problem was with a change made to the show_ads.js file that is downloaded for every AdSense ad unit view. The problem was first reported on August 11th, but it took until August 15th for the problem to be resolved. More details on the specifics of the problem at JenSense.
Optimising Digital Marketing Campaigns with Search, Social and Analytics
At SES London (9-11 Feb) you'll get an overview of the latest tools, tips, and tactics in Paid, Owned, Earned, Integrated Media and Business Intelligence to streamline your marketing campaigns in 2015. Register by 31 October to take advantage of Early Bird Rates.