On the basis of algebraical results examplified with exact computations, we investigate the dynamical effects associated with the presence of the surface potential barrier in very low energy electron diffraction (VLEED). These effects essentially have three origins: (i) the diverging phase of the reflection amplitude from metal back to metal leads to rapid oscillations of intensity located below emergence thresholds, the maxima of which are arranged in a Rydberg-like series. This effect, wrongly attributed to a resonance process, is actually due to a simple interference mechanism, (ii) The singularity near zero of the transmission amplitude of the barrier from metal to vacuum leads to a discontinuity in the current of non-specular beams at their emergence threshold, (iii) The interference between the waves directly reflected at the surface barrier and at the atomic planes generally induces large modifications of the intensity profiles spreading out on the whole VLEED energy range. Thus the approximation consisting in neglecting the reflection at the surface barrier is inadequate. In conclusion, the above barrier effects must be properly included in any quantitative analysis of VLEED spectra.