When your friend promises to save you the last piece of gum but doesn't, you might curse her for her inconstancy. Inconstancy means changeability, not sticking to a predetermined course.
Inconstancy is a word often associated with love––if someone promises to love you forever in eighth grade but ends up falling for someone else in ninth, their behavior testifies to the inconstancy of their love. Or the inconstancy of youth. Or maybe just the inconstancy of love in general.