One of our Xserves was recently connected to a new KVM via a Mini DisplayPort to VGA adapter. When connecting the Mini DisplayPort to the Xserve, the video signal was not usable (screenshot below).
A reboot will clear the issue, but why reboot a production server? Especially this server with over 425 days of uptime!
The fix is to use SSH with the following command:
sudo killall -HUP WindowServer
This will log the user out, and reset the video. Once reset, all worked again as expected.