On a related note, Knight Capital lost $440 million due to a software bug a few years ago.
http://www.bloomberg.com/news/articles/2012-08-02/knight-has...
http://www.bloomberg.com/news/articles/2012-08-02/knight-has...
The SEC order there is an interesting read for anyone involved in large scale transactional systems: https://www.sec.gov/litigation/admin/2013/34-70694.pdf