Advertising news sources are saying that it was an accident.

On Tuesday at about 7 p.m. ET, many publishers both in the U.S. and Australia saw many–if not all–of their ad slots filled with display ads featuring nothing but the color yellow. They were up for 45 minutes.

The costly mistake occurred during a Google training program when an employee accidentally purchased the 300×250 ad units, the sources said. Publishers who checked their logs saw the advertisements came from theiconic.au.com, an Australian retailer.

Estimates are upwards of $1m burned in just hours. Google made an official statement, confirming both payments and that the protest didn’t encounter any resistance.

We will honor payments to publishers for any ads purchased and are working hard to put safeguards in place to ensure this doesn’t happen again