  It has recently become popular to analyze the behavior of excess dislocations
in plastic deformation under the assumption that such dislocations are arranged
into walls with periodic dislocation spacing along the wall direction. This
assumption is made plausible by the fact that periodic walls represent minimum
energy arrangements for dislocations of the same sign, and it allows to use the
analytically known short-ranged stress fields of such walls for analyzing the
structure of plastic boundary layers. Here we show that unfortunately both the
idea that dislocation walls are low-energy configurations and the properties of
their interactions depend critically on the assumption of a periodic
arrangement of dislocations within the walls. Once this assumption is replaced
by a random arrangement, the properties of dislocation walls change completely.
