Today, I had ssh'd into a a server and I tried to launch a X11 program, and I got this error message:
X Error of failed request: BadAtom (invalid Atom parameter)
It looks like that Mac OS X 10.4 includes version 3.8 of OpenSSH. This new version defaults to using untrusted cookies for X11-Forarding. This can be worked around by using 'ssh -Y' instead of 'ssh -X'.
[via macosxhints Forums]