In the plasma sheath the electron current is severely limited by a retarding electrostatic potential, which implies that only fast plasma electrons reach the material surface. These fast electrons have a much longer mean free path than thermal electrons and consequently their last collision will have occurred well away from the surface. If the temperature gradient is such that the temperature at these last collisions is different from that at the surface, the sheath potential will be modified. This effect can lead to large modifications of the sheath potential and possibly also to substantial errors in the interpretation of probe signals.