Knight Capital - They were biggest equities trader in 2012. They manually deployed code and didn't get configuration right and it reactivated "Powder Peg". They lost $460 M in 45 minutes and went bankrupt.
The program was called "Power Peg" for those googling for it. It was a test program not intended to be used on the live market.
The Power Peg program was designed to buy a stock at its ask price, and then immediately sell it again at the bid price, losing the value of the spread.
Thanks for sharing. This is exactly the kind of blunder I had in my mind when asking the question, a seemingly silly mistake like forgetting to do something causing way too much trouble!