Imagine if every Thursday your shoes exploded if you tied them the usual way. This happens to us all the time with computers, and nobody thinks of complaining.
My definition of an operating system: What you have to hassle with before you get to hassle with the application.
Picture yourself driving your car. Let’s say that every Thursday the gas pedal and the brake pedal are interchanged. It would drive you up the wall, or through it. You couldn’t live with it. Yet our computers change things around all the time by using “modes”. A system should be “modeless”. The same user action should always have the same effect.