In close succession to our own investigative bug analysis Wired have recently published an article entitled ‘History’s Worst Software Bugs’ (definitely worth a quick glance).
Here’s a quick quotation from the source:

What engineers didn’t know was that both the 20 and the 25 were built upon an operating system that had been kludged together by a programmer with no formal training. Because of a subtle bug called a “race condition ,” a quick-fingered typist could accidentally configure the Therac-25 so the electron beam would fire in high-power mode but with the metal X-ray target out of position. At least five patients die; others are seriously injured.

See the original for more.
Bugs like these would require their own priority level in Jira – ‘Imminent Fatality’ …

… and some bugs are just life threatening