Knight Capital lost control of its trading in 45 minutes

The story

Knight Capital was one of the largest stock traders in the United States, handling a large share of trades through automated systems.

For a new trading program, a technician installed updated software on Knight’s order routing servers. There were eight servers, and the update was copied to only seven.

The new software reused a setting that, years earlier, had switched on an old function no longer in use. On the seven updated servers, the setting did what was intended. On the eighth, it woke up the old code. That code had been changed over time so it never recorded when an order was complete, so it kept sending more.

When trading opened on August 1, 2012, the eighth server began flooding the New York Stock Exchange with orders. In about 45 minutes, Knight made millions of trades across 154 stocks. The losses were so large that the firm agreed to be taken over by a rival before the end of the year.

Why it mattered

By the next day, about three-quarters of Knight’s value had been erased, starting from one server that missed an update. It is one of the clearest examples of how a single missed step in a software change, with no check that every machine matched, can sink a company.

The lesson today

Knight did not fail because of one mistake. A manual update missed a machine, old code was left in place, and nobody had a quick way to stop it. Updates should be rolled out consistently and checked, old software should be removed, and there should be a clear way to roll back.