Yesterday, for about five minutes during the day, any site that had AdSense ads on them, showed what is known as Public Service Announcements (PSAs). PSAs are ads that are free and don't earn money, and are shown when Google cannot find a contextually relevant ad.
AdSenseAdvisor confirmed the issue in a WebmasterWorld thread, saying:
We experienced a very brief (under 5 minutes) issue that caused Public Service Announcements (PSAs) to appear on publisher pages for a few minutes this afternoon. Our engineers diagnosed and resolved the issue immediately, and your ads should now be back to normal. Thanks for reporting this, and we appreciate your patience.
This also seemed to impact publishers who use Google Ad Manager to serve up their ads (which is what we use on this site). Steve from the Google Ad Manager team said in a Google Ad Manager Help thread, "We experienced a brief issue that caused PSAs to appear on publisher pages for a few minutes this afternoon. Our engineers quickly diagnosed and resolved the issue, and your ads should now be back to normal. Thanks for reporting this, and we appreciate your patience."
This is not the first time PSAs invaded the bottom-line for publishers. We reported a similar issue back in 2004 and also earlier this year, in January.
Forum discussion at WebmasterWorld & Google Ad Manager Help.