  We compute the time evolving probability of a Gaussian wave packet to be
reflected from a rectangular potential barrier which is perturbed by reducing
its height. A time interval is found during which this probability of
reflection is enhanced (superarrivals) compared to the unperturbed case. Such a
time evolving reflection probability implies that the effect of perturbation
propagates across the wave packet faster than its group velocity - a curious
form of nonlocality.
