If a window requests a large size (mozilla being a common villain)
then metacity clips it to screen size but fails to clip it to avoid
panels at the top or bottom of the display. Faced with such a problem
a novice user cannot hope to sort out their desktop, and its a pain
for poweruser too.

Metacity should be clipping the window to drop within the panel range.
It should also reclip window sizes when panels move and are resized
but that in itself is much less pressing than the basic problem.