I owned a pay per click management agency from 2005-2009. In 2008 I decided to buy some Enterprise software to help me better manage big clients in competitive spaces. I migrated these clients over to the software, set up a bunch of automated rules to change bids and called it good. Then Saturday hit and we suddenly exceeded a single account budget by $40,000.
That was the day that I lost $40,000. I took responsibility for every penny of that loss.
After a thorough internal investigation, we determined it was a bug in this Enterprise software. We took our records to the software company and they admitted or confirmed that it was indeed a bug on their side. Unfortunately, they were such a big company that we couldn't get anywhere when seeking a refund. They lawyered up real fast upon our request for our money back. I didn't have the means to pursue it at that point, so we took the hit as a business and continued on our way.
The lessons I learned: audit every automated task, find ways to lower your risk while implementing changes, and hire/retain a business lawyer for the duration of your business.