Ah, the old crossed wiring problem.While you might think this would be easy to catch, it obviously isn't.It is good that they were able to diagnose and fix the problem without any long term consequences.
When I was working on satellites we were warned about the reversed sign problem in attitude control systems.Surprisingly one got by the extensive testing and the satellite was sent into deep space instead of into near earth orbit (oops!).
Now that is a Sherlock Ohms tale I can relate to. How many times does the root cause of the problem end up being something so basic, but likely often overlooked because you're searching for the complicated problem.
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