Many things

[Tony Duell]

> >Of course there is the big difference that correctly working hardware can 
> >fail (a component can go out-of-spec), correctly working software 
> >doesn't. But anyway...
> >
> Don't be so sure.  I know of one bug in RT-11
> which has been there for over 30 years.  Because

No, I stand by what I said. If there's a bug, then that software is not 
correctly-working (even if it seems to be).

The analogy for hardware would be a marginal timing problem (wher,
peerhaps, every 10 years 2 signals arrive effetively in the wrong order).
That's a mis-design, it's not working correctly. 

But correnctly-working software carries on working. Hardware on the other 
hand can stop workin if a component fails.

-tony