A WebmasterWorld thread shares with us that an advertiser noticed his client's credit card was hit up 42 times for a charge of $1,000. So he was billed 42 time $1,000, totaling $42,000.
YahooSarah, the official Yahoo! Search Marketing Representative, came in to explain:
Yahoo! experienced a technical issue that caused the credit cards of a few advertisers to be charged incorrectly on Wednesday, February 14. Some credit cards were authorized for one or more charges in error.
We caught this early and were able to halt the process before the charges were actually completed. We’re very sorry for any issues this might have caused you and we're doing everything possible to completely resolve the situation as quickly as possible, and to ensure that it doesn’t happen again.
Please keep in mind that during a credit card transaction such as this, an "authorization hold" is placed against the card (similar to when you check into a hotel, and a hold is placed against your card for potential incidental charges). These authorization holds will automatically be removed from your credit card account, usually within three to seven business days. If you are finding that the authorization hold is causing you inconvenience in any way, we encourage you to contact us at 866-YAHOO-SM (866-924-6676), and we will be happy to assist you in contacting your credit card's issuing bank to manually remove the authorization (that said, a bank may take as long as 48 hours to completely remove the authorizations).
So, the cards were not actually charged yet, but they had "authorized holds" on them that meant if this person's spend limit was $42,000, he would not be allowed to charge more items on his card until the authorization was cancelled or paid in full.
Yahoo! did say that they "automatically be removed from your credit card account," and if they have not been removed, you can contact Yahoo!
This was a serious, serious bug and it doesn't seem all that wide spread. Kudos to Yahoo! for being upfront about it.
Forum discussion at WebmasterWorld.