side effect

sivuvaikutus puhekieltä An unintended consequence of any action in addition to the intended consequence of that action.
(quote-book)
puhekieltä An adverse effect, an unintended consequence of a drug or therapy; usually not a beneficial effect.
puhekieltä A change in state caused by a function call (typically "side-effect").

Eiffel is somewhat purist about insisting that functions have no side effects.