We worked with a payment processor to implement billing for our services via credit card. According to the payment processor, the QA environment for one of the major credit cards had been broken for a while, so we tested in production.

We were testing billing customers who were going to pay us, so putting a small charge on a corporate card that was going to come back to us wasn't a big deal, I just remember being slightly surprised that testing something like credit card payments was done against the production environment.