X Window

It never happened to me under Red Hat 9, but when I am working in KDE as a normal user and try to execute emacs from a terminal, in which I am su, I occasionally get the following error message:

jeltsch@mcblpc2:~> su
Password:
mcblpc2:/home/jeltsch emacs test.txt
Xlib: connection to ":0.0" refused by server
Xlib: Invalid XDM-AUTHORIZATION-1 key (failed key comparison)
emacs: Cannot connect to X server :0.0.
Check the DISPLAY environment variable or use `-d'.