Briefly:
	 :	Patti Thornton, a senior technical engineer in the shuttle group, explains the most remarkable thing about the error and its paper trail. "There is no starboard manipulator arm," she says with a smile. "The code is set up to accommodate one, but there isn't one now, and there's never likely to be one."

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Seems to me they added complexity for the starboard manipulator arm when they didn't need it. Hence, they have more bugs (from duplicated code) that they can't find (because it isn't used). AirplaneRule squared. -- SunirShah

''Wouldn't you have to go out of your way to design a side without an arm and a side with an arm, vs. two instances of a side that accommodates an arm?''

Perhaps there were going to be two arms, then the budget got cut.  The code worked, IfItAintBrokeDontFixIt.

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Plus that was 70d technology. In the 21st century we have ... the Special Purpose Dextrous Manipulator - denizen of the SpaceStation and perhaps soon-to-be-rescuer of the HubbleTelescope - see http://www.chron.com/cs/CDA/ssistory.mpl/front/2780175 (MD Robotics makes both the Canadarm and the Manipulator, which is 2-handed)
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It's in case they have to land in Australia, where everything is reversed.