I'm not 100 % have I categories this correctly, but lets try.
OS: Fedora 18
Steps to repro:
1. Sign in as User A
2. "Switch user" and sign in also as a User B
3. Wait until screen gets locked
4. Try to sign in as a user A again.
The problem is, when two users are sign in into my system and I would like to switch back to the user signed as first, I'm able to do so.
That would be a problem with the implementation of whatever desktop
environment you are using (GNOME, KDE, XFCE, etc.), not with the
specification of the X11 standards, so this is not the right place
to report it, sorry.