I realize that this is not an issue in general, but replying in private message to someone who is using the nickname "/quit" causes mIRC to quit and close all channel windows.

Uhm... here nothing strange...

I don't know which network you are on, but / should *never* be allowed to part of the nick. RFC 1459 specifies which characters are allowed, and if said network is breaking that, then there's very little mIRC (or any other client) can do.

starfox is right
altgought i can't see the reason this is happening...
a possible decision can be the changing of the command prefix -> Options -> General -> Command prefix

You're right about changing the command prefix, but that would only affect the person who'd changed it. I can't see how anyone could use the nick "/quit" without getting an 'erroneous nick' message. And if you used /msg /quit or /notice /quit wouldn't you get a message 'no such user', rather than getting disconnected?

if the network supports / as nickname char why not?;)

Fair enough, but it still doesn't make sense. If "/" was supported as a nick character, then "/" wouldn't be a command prefix would it? Therefore typing /quit wouldn't disconnect you...ugh, I'm confused now

right, but he says that he got disconnected when he sends message, thats PRIVMSG /quit something, and on any normal network it will just result in raw 401 - /quit :No such nick/channel
oh, there's no way to test that with the "/quit" gyu present, at least not on any network I know
whatever ;b

I need a drink!

If an IRCd does support that character then it's just plain stupid of the creator of it.

I didn't said the opposite!
I was just thinking about what exactly will cause this
anyways

Suppose /quit is a valid nick, I don't understant why mIRC would send this as a command. Unless you specifically time in the nick of the person at the beginning of the line, than how would mIRC get this as a command??