It appears that if OS X cannot locate your home directory where the NetInfo database says it should be, it will create the path under /Volumes. So, assuming an installation with two hard disks, in which the OS is on DiskA, the Users folder is on DiskB, the home directory path for user Foo should look like this:
/Volumes/DiskB/Users/Foo
But if something goes wrong during startup, and that path does not exist, then the path will be created in the Volumes directory. What happens next is not entirely clear to me –- but I think when DiskB then does become available, the attempt to mount it normally fails, as the path has been created in /Volumes, so DiskB is mounted at /Volumes/DiskB 1.
You then appear to be logged into your normal account, but you are not. Your real home folder (with all its contents) will be mounted at /Volumes/DiskB 1. You can confim this by command clicking the icon of your home directory in the window's title bar. The path will be shown as :
Disk A .Volumes DiskB Users FooThe best way to recover the situation is to login with a different installation (eg Mac OS 9, or the startup CD). Delete the symlink to the new Users folder, and rename /oldUsers to /Users. You're now back to a standard installation; startup as normal. For safety, rename DiskB to something else, say DiskC –- this will avoid you accidentally wiping your Users folder when you sudo rm in the next step.
Now cd into the /Volumes directory, check the contents of DiskB (it should contain only the new, near-empty /Users/Foo). Once you're sure it's not your real home folder (that should be safely on the renamed DiskC), then do sudo rm –R to send the impostor to bit heaven.
Rename DiskC back to DiskB, and symlink /Users to /Volumes/DiskB/Users, rename /Users to /oldUsers, and you're done. If any *nix wizards can shed more light on this, I'd be grateful.

