Joining Other Player's Server By Request

Note To All users:
"There is no such thing as a force op hack. Well, to be fair, there isn't one that has been proven to work. The latest one, which has been known for a few months now, is a session stealer where the griefer will try to have someone log into a "trapped" server that will use the session ID to log into the target server.
There isn't any way to prevent this at this time, besides not trying to connect to unknown servers when asked by someone you do not know, it's a bug in the way Mojang handles authentication of accounts. Mojang is aware of the hole and, last I heard, it will be fixed in the next release of minecraft."