  The dynamical effects inducing geometrical and phase space misalignment of
bunch slice in X-ray operating Free Electron Lasers can be traced back to a
plethora of phenomena, both in the linac accelerating section or inside the
beam transport optic magnet. They are responsible for a spoiling of the beam
projected qualities and induce, if not properly corrected, an increase of the
saturation length and a decreasing of the output power. We discuss the
inclusion of these effects in models employing scaling formulae.
