When you do /spectate [ID] when that person is fighting, Then do /spectateoff
Then you are in a world and you cant do a CMD unless you type /kill (otherwise it says: /leave or else finish the match)
When you type kill then it says: [name] has lost the fight! he ***** ****
Its sometimes anoying.

I've also found this problem.
Also, when you are spectating and somebody accepts the challenge that you're in, you'll get in that world.