Sandwich gravitational waves exhibit the memory effect (VM) which however can become, for specific values of the wave parameters, pure displacement (DM) as suggested by Zel'dovich and Polnarev. Fixing such a "miraculous" value, the particle trajectory is an (approximate) standing wave characterized by a unique integer $m$, for which the particle does not absorb any energy from the passing wave. Our statements are illustrated by a simple Gaussian and by the Pöschl-Teller potential as profiles.