Rule of least surprise

http://en.wikipedia.org/wiki/Principle_of_least_astonishment

A user interface may have the behaviour that pressing Control-Q causes the program to quit. The same user interface may have a facility for recording macros, a sequence of keystrokes to be played back later, intended to be able to control all aspects of the program. The user may want to record a keystroke sequence that includes Control-Q as part (most likely the last part) of the macro. The principle says that pressing Control-Q while recording a macro should not quit the program (which would surprise the user), but rather should record the keystroke.