[Quarrelsome]

You also have the free will of humans messing with the system (in my case, fraud prevention), also consider animals interacting the machinary while someone is not watching. I remember hearing a story about a huntsman spider regularily tripping a sensor for a program that measured the weight of lorries.

The only consistent thing you can state is that all these "real world" issues are to do with hardware whereas yours are more softwarey. However the consideration of such issues is necessary across the board.