Everytime I go to spectate a game. It tells me I've suddenly disconnected and can either reconnect or quit. If I try to reconnect same thing happens.
Same with me tooo
Its been widely reported on the forums, and seems to only happen to people running 10.6.8. Upgrading to 10.8.3 has made spectator mode work for several people.
I'm sure they'll fix it under 10.6.8 eventually, but don't expect it anytime soon.
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