Hi Tiziano,
Aha! Problem found and fixed.
I got the smaller file via private e-mail (Yahoo strips attachments).
Not long ago, I added a new overlay line-drawing mode to Guide, that
allows one to draw a line, then add points to create a polygon; see
http://www.projectpluto.com/update8.htm#ovl_lines
A fine and wonderful thing, indeed... except that I didn't get it
quite right; I made a really dumb mistake that leads, in one case out
of 256, to a possibility of mangled lines of the sort you saw.
I've posted Yet Another Bug Fix update, again at
http://home.gwi.net/~pluto/guide8b.zip
which includes this fix.
-- Bill