Were the trades any good though?

No, it was caught by our trading ops guys. A few minutes after I hit enter I got. Rather chilling phone call from them. So that part of the system worked

Plot twist: It made so much money that that’s now their strategy.

Back in school, my roommate's mom worked for a hedge fund and he did part-time work for them. He factored out a common trading engine from individual strategies, and one day the head of the fund asked him to run a strategy that had made a bunch of money in the past, but had been retired after failing to make money for a while. So, he put the strategy back in production without any testing, forgetting that he had recently done some minor refactoring of the trading engine. He typo'd one variable name for a similar variable name, so in the loop where it broke down large orders into small orders, it actually had an infinite loop. Luckily the engine had an internal throttle, so it wasn't trading as fast as it could send messages over the network.

I was chatting with him when he noticed the stock the strategy was trading (KLAC) was gradually declining linearly. He looked at the L2 quotes and saw that someone using his brokerage was repeatedly putting out small orders, and then he realized they were his orders.

The fund got a margin call and had to shift some funds between accounts to make margin, and they had to contact regulators and inform them of the bug, and they had to manually trade their way out of the massive short position they traded. However, they ended up making $60,000 that day off of his mistake.

This is such a cool story.

That's an excellent postmortem culture.