This happened to me once. I was not allowed in the building so I called a colleague to bring me in as a visitor. Upon trying to login, "account has been disabled". Called IT, "you've been fired". Called my line manager, "I have no idea what you're talking about". Turns out someone forgot to extend my contract on the system.
Didn't Twitter do something like that recently?
Although, begrudgingly I agree that it's something that should probably be done if you know somebody is being let go.