joining channel 0 is _almost_ like an irc statndard. When done, it parts you from all channel names. The syntax for channel lists are [#|&]Channel[,[#|&]Channel]
Where you have a channel beginning with # or &, then a comma seperating it. When a channel 0 is listed, you re parted from all. I would almost suggest that mirc offers a "Catch" for this as it has for links now. "You have chosen to join channel 0. You may be parted from all channels if you continue. Would you like to join channel 0?" That way just incase one seems to sneak by you, you can catch it >:D