"We think the two periods of time when there was a sudden drop in trading (9:48 and 9:52) are when they restarted the system. Once it came back, the Tester, being part of the package, fired up too and proceeded to continue ..."
Worse dumb mistake I've ever made was to accidentally send a test email to several thousand live customers instead of the test accounts. That was a sinking feeling.
But creating a bug that loses your company half a billion dollars in thirty minutes and bankrupts them, must be stomach-churning.
Much, much worse than the bug in the market making algorithms was the design failure of not having some out-of-band mechanism to kill order traffic (or of said mechanism's failure to be tested adequately).
This is a risk management failure much more than a programming error.
Ouch!