It is allowed to try and allocate a pixmap which is larger than
  32767 in either dimension. However, all of the framebuffer code
  is buggy and does not reliably draw to such big pixmaps, basically
  because the Region data structure operates with signed shorts
  for the rectangles in it.

  Furthermore, several places in the X server computes the
  size in bytes of the pixmap and tries to store it in an
  integer. This integer can overflow and cause the allocated size
  to be much smaller.

  So, such big pixmaps are rejected here with a BadAlloc

  Originally contributed by FreeNX Team