Mountain Lion displaying graphical artefacts on screen I have a default automatic login account, User A. The Dock renders all it is responsible for (Dock, Launchpad, Mission Control) correctly for User A, but when I Fast User Switch to User B or C or the Guest Account, the Dock process doesn’t render anything properly, as shown in the screenshot.

This doesn’t happen when I disable automatic login and login to either User B, C or Guest on boot.

I did not have this problem on Lion.

If it matters, I have an Sapphire ATI Radeon 5450 HD.

Why is this happening and how do I fix it?
Check for updates through the Mac App Store first just in case Apple may have fixed this (obviously not everyone is experiencing this issue, but they may have anyway).  In the comments of this article about Mountain Lion bugs, many users are expressing the same issue that you are having, and having to logout and back in again to fix this.  

Another thing you might try (if you haven't already) is to back up and re-install Mountain Lion.  

update Everyone on this thread has the same exact issue. Yes, it is a bug, and yes, the only workaround is to killall Dock. I suggest using an AppleScript with Login Items, launchd, or Lingon (old, free, unsupported version here, and $3 newer, supported version here) to run killall Dock as soon as you log in so that the Dock just restarts itself immediately.  Meanwhile, submit this bug to Apple.