I do not have an insulin pump and I'm presuming you don't with my response. The pumps I have seen make noise when they are pumping and also have an alarm (I'm not sure if that's from the pump or the reader) if your levels are on either extreme. I'm guessing malfunctioning code would only execute once by pumping too much or not pumping at all and the user would figure it out very quickly.
I agree about the importance of medical devices but I don't think diabetic coma happens as easily as you think for most (if they are on one of those full-systems). From what I've seen from insulin pumps/systems:
1. There is a max rate that it can pump
2. There are alarms that go off if the numbers are above or below your settings (I believe there are also max and mins for those so you can't accidentally set it to a 'fatal' number)
3. The systems seemed 'independent.' The reader did not take for granted that the pump was pumping, but rather continually monitored glucose levels and would still set off an alarm if the numbers weren't within the bounds.