I've got a Debian Sid box running XFree86 (debian package 4.3.0-7).
If I pipe emacs -q from it (emacs 21.3 or CVS, doesn't matter) to my
WinXP box running XWin 6.7.0.0-1, click the window to get rid of the
Emacs intro screen, and select text right to left, Emacs crashes with
X protocol error: BadWindow (invalid Window parameter) on protocol request 38
I assume "pipe" means ssh tunnel. If so, see A1 in this FAQ entry:
http://x.cygwin.com/docs/faq/cygwin-x-faq.html#q-ssh-no-x11forwarding
Harold
