It seems like if you spectate a arena, and a player inside it leaves the arena, you can still teleport to the player and spectate him (but it does say in the GUI "You and X is not in the same world")
This should not be possible.

I can't test that right now, but I could Imagine that cause it. A rough method to prevent this, would be to first check if the player is inside the same world as the arena, which would eliminate a lot of exploits :)

Yes indeed. I wasn't thinking about multi world support when I added arena
mode ages ago...

Np, it's a common mistake among developers, that they forget multiwords :)

Does the player out of the arena bounds happen to be inside the same coordinates as the arena bounds, but in the other world?