PLS

BOO

...This is an example of poor interface design. You didn't expect a BOO did you?

Principle of Least Surprise

A user-interface design idea, that being that when a user clicks/types/whatever to the computer, the computer should do the most logical/unsurprising thing from the users point of view.

This point has to be made obvious because sometimes the users expectations do NOT match the expectations of the system designer, or the programmer, etc.

Note that this is good design from a computer systems point of view, but makes for poor entertainment. Peek-a-boo, for example, would be dull without surprises...