Points out a certain necessary condition for a detached magneto-hydrodynamic shock to form in front of a magnetic field which acts as a piston moving a collisionless plasma. This condition takes the form of a requirement that the time during which the piston accelerates to the Alfven speed be greater than a certain value determined by the plasma parameters. Unless this condition is met in an experiment a detached shock will not form, so that any interpretation of the observations which uses shock theory will be incorrect.