Fundamental Bug - 4 Years of Bad Stock Trades

Not exactly a security issue, but it ought to be.  The third largest US stock exchange operator had a systematic bug in its trading software. It was undiscovered for over 4 years and it cost their clients over $400k - not a lot of money relatively speaking.  But it's been there for 4 years!!

http://www.bloomberg.com/news/2013-01-10/bats-says-system-errors-caused-pricing-problems-over-4-years-1-.html

How could this not be found in testing! What does their software life-cycle look like? What other undiscovered bugs are lurking in their system?