pin Just out of curiosity, why should they be messed up?
As I mentioned above, I faced similar issues with a NFS-shared home, as I was using the same hostname/username combination across the various client systems. Also if I remember correctly, changing the hostname will confuse xauth, since the X11 client library will no longer be able to find a matching cookie inside .Xauhority, resulting in a rejected connection1.
Finally, if you're working as root in your user's HOME and launch a X session or you run startx with sudo, you'll generate a root-owned Xauthority. This is the sort of mistake one may make as *nix beginner,; in my case, it happened several years ago.
1 and since @pfr had been changing the hostname associated with the loopback address inside /etc/hosts, I thought that to be the root of the X11 hanging problem.
pin I'd expect even more trouble when .Xauthority doesn't exist or, am I just confused?
Not quite; since the startx script already takes care of invoking xauth to generate a random authorization cookie and store it inside .Xauthority as a MIT-MAGIC-COOKIE-1 entry, using startx or xdm to lauch a local X11 session should always be enough to get credentials right. Things get trickier when launching a session remotely with Xnest or Xephyr over SSH or XMDPC, since you have to that manuallly.