The principle of least astonishment (POLA/PLA) applies to user interface design, software design, and ergonomics. It is alternatively referred to as the rule or law of least astonishment, or the rule or principle of least surprise (POLS).

The POLA states that, when two elements of an interface conflict, or are ambiguous, the behaviour should be that which will least surprise the user; in particular a programmer should try to think of the behavior that will least surprise someone who uses the program, rather than that behavior that is natural from knowing the inner workings of the program.

Interesting.