  Imperfections of Bunimovich mushroom Billiards are analyzed. Any experiment
will be affected by such imperfections, and it will be necessary to estimate
their influence. In particular some of the corners will be rounded and small
deviations of the angle of the underside of the mushroom head will be
considered. The analysis displayed some unexpected non-generic features. The
latter leads to a transition from a perfect mushroom behavior to either an
ordinary KAM scenario or an abrupt transition to complete chaos, depending on
the sign of the perturbation. The former produces a fractal area of islands and
chaos, in fact a KAM scenario, not associated to the large island of stability
of the mushroom billiard.
