Anyone out there have good examples of sites that limit the number of
concurrent users at a given time?
For example, if the restriction is set to a single user per account
at a time, then two users cannot access the site with the same
account during the same period of time.
I'm interested in the experience each user faces (doesn't face)
when the second log-in is attempted, and what each user might
experience as a result of that attempt.