i'm really not crazy about how mIRC handles /list; how we're not able to control the list window, how $line() is inaccurate with it, etc. it's not a great solution, since /list performs a little handling before sending a request to the server, but you can use /raw list <parms> to send a request directly to the server and override the mechanism that prepares mIRC for receiving the list of channels. that way, if the server sends back an unrecognized response, nothing will have changed internally :P
