Why it matters: Anyone who has been in training for a new job knows how scary it can be, constantly worrying about making a mistake that could get you in trouble. So, spare a thought for the Google trainee who accidentally published a dummy advert that's thought to have cost the firm around $10 million.

The mistake happened during a group training exercise in which Google employees were shown how to use the company's in-house ad placing system. The Financial Times reports that one worker "went further than intended" and submitted a live buy order into the system.

The error caused a fake advert, made up of nothing more than a yellow rectangle, to appear in numerous websites and apps across the US and Australia for around 45 minutes on December 4. "As soon as we were made aware of this honest mistake we worked quickly to stop the campaigns running," said Google, in a statement.

If that doesn't sound bad enough, the trainee placed the order at around ten times the normal price for the ads. At $25 per CPM (cost per thousand impressions), it was much higher than the usual $2 - $4 market price.

Not only was the ad placed on Google's AdX, but it was also placed on several ad exchanges, which helped ensure it reached a wide audience. Google has promised to "honor payments to publishers for any ads purchased." It never revealed exactly how much this mistake cost the firm, but the FT believes payments to publishers and cleanup bills put it at around $10 million.

Google says it now has safeguards in place to ensure such a costly error never occurs again. No word on what happened to the employee.