I began encountering errors after installing Tiger when using an X11 forwarding session on my Mac to a remote machine using ssh -X, and then trying to run remote X-apps.
To overcome the errors, I found I now have to use the -Y option instead of the -X option with ssh to start X11 forwarding properly. The -Y option is the same as -X, but is more strict about security. From the man page:
-X Enables X11 forwarding. This can also be specified on a per-host
basis in a configuration file.
X11 forwarding should be enabled with caution. Users with the
ability to bypass file permissions on the remote host (for the
user's X authorization database) can access the local X11 display
through the forwarded connection. An attacker may then be able
to perform activities such as keystroke monitoring.
-Y Enables trusted X11 forwarding.
Thus, Tiger seems to be more strict about X11 forwarding than was Panther...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050504114932249