Therac-25
The Therac-25 was a radiation therapy machine produced by Atomic Energy of Canada Limited (AECL) after the Therac-6 and Therac-20 units (the earlier units had been produced in partnership with CGR of France). Therac 25 user interface
It was involved in at least six accidents between 1985 and 1987, in which patients were given massive overdoses of radiation
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2003 North America Blackout
The blackout's primary cause was a software bug in the alarm system at a control room of the FirstEnergy Corporation in Ohio. Operators were unaware of the need to re-distribute power after overloaded transmission lines hit unpruned foliage. What would have been a manageable local blackout cascaded into widespread distress on the electric grid.
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Mars Climate Orbiter
On September 23, 1999, communication with the spacecraft was lost as the spacecraft went into orbital insertion, due to ground based computer software which produced output in non-SI units of pound-seconds (lbf×s) instead of the metric units of newton-seconds (N×s) specified in the contract between NASA and Lockheed. The spacecraft encountered Mars at an improperly low altitude, causing it to incorrectly enter the upper atmosphere and disintegrate.
Don't forget the Ariane rocket disaster: http://en.wikipedia.org/wiki/Cluster_(spacecraft)#Launch_fai...
This one is my favorite: http://thedailywtf.com/Articles/_0x2f__0x2f_TODO_0x3a__Uncom...
Another incident we went over in the ethics section of one of our software development classes was one where an automated AA gun killed 9 and wounded 14 in South Africa.