He was just humming through his day like one of those little robotic vacuum cleaners, and probably thinking about pickled things that really shouldn’t be, and bothering nobody, when this suddenly dawned on him:
The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.*
Then the humming resumed.
* Douglas Adams