  Particles initially at rest hit by a passing sandwich gravitational wave
exhibit, in general, thevelocity memory effect (VM): they fly apart with
constant velocity. For specific values of the wave parameters their motion can
however become pure displacement (DM) as suggested by Zel'dovich and Polnarev.
For such a ``miraculous'' value, the particle trajectory is composed of an
integer number of (approximate) standing half-waves. Our statements are
illustrated numerically by a Gaussian, and analytically by the P\"oschl-Teller
profiles.
