To build such a crash-resistant system, the designer must be able to imagine - and disallow - the dumbest action. He or she cannot simply rely on the user's intelligence: who knows who will be on the other side of the program? Besides, teh user's intellligence is not quantifiable; it's not programmable; it cannot protect hte system. The real task is to forget about the intelligent person on the other side and think of every single stupid thing anyone might possibly do.
In the designer's mind, gradually, over months and years, there is created a vision of the user as imbecile. The imbecile vision is mandatory. No good, crash-resistant system can be built except if it's done for an idiot. The prettier the user interface, and the fewer odd replies the system allows you to make, the dumber you once appeared in the mind of the designer.
The designer's contempt for your intelligence is mostly hidden deep in the code. But, now and then, the disdain surfaces. Here's a small example: You're trying to do something simple, like back up files on your Mac. The program proceeds for a while, then encounters an error. Your disk is defective, says a message, and below the message is a single button. You absolutely must click this button. If you don't click it, the program hangs there indefinitely. [...] You must say, "OK."
relevant to PEBKAC