Lately I've been occasionally getting the following error:

Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key
Error: Can't open display: :0.0

sometimes the last line is:
(null): XOpenDisplay failed

I got this when trying to run xfishtank, and when trying to run
the mwm or twm window manager.  Both of these worked previously.
It seems to happen more often if I am root when I try to launch
the app.

It's not a big deal, but it is puzzling.

thanks,
Roger