On August 1, 2012, Knight Capital, the firm behind about a tenth of all U.S. stock trading, deployed new code to seven of its eight order-routing servers. The eighth still had code the company had retired in 2003, behind a feature flag the new code reused. In 45 minutes it executed 4 million trades and left Knight with a loss of more than $460 million. The best postmortem of it was written by the regulator, and every lesson in it still applies to anyone who ships with feature flags. ...