  We investigate in a model with scalar ``nucleons'' and mesons the
contributions of higher Fock states that are neglected in the ladder
approximation of the Lippmann-Schwinger equation. This leads to a breaking of
covariance, both in light-front and in instant-form Hamiltonian dynamics. The
lowest Fock sector neglected has two mesons in the intermediate state and
corresponds to the stretched box. First we show in a simplified example that
the contributions of higher Fock states are much smaller on the light-front
than in instant-form dynamics. Then we show for a scattering amplitude above
threshold that the stretched boxes are small, however, necessary to retain
covariance. For an off energy-shell amplitude covariance is not necessarily
maintained and this is confirmed by our calculations. Again, the stretched
boxes are found to be small.
