Normally, chance is identified as an accident.
And accident is a transient, non-essential property of a thing or process, as opposed to what is essential, necessary and substantial. But if unexpected is a chance, we can't consider it as an accident too...
Denial of the objectivity of necessity, the idea that historical events are the outcome of the chance occurences and individual actions, etc., is associated with Voluntarism. For dialectics, Chance and Necessity are inextricably linked. Necessity asserts itself through the interaction of millions of accidents, while each such ‘accident’ is the outcome of a necessary sequence of causes.
For metabologics, unexpected is not a chance nor a necessity. Its concept is strictly opposed to possibility... or im-possibility.