Starting Oct 12, 2003, an exploit was used to crash many people's mIRC clients. All versions from 6.0 thru the recently released 6.11 are affected. (It does not appear to affect version 5.91 or earlier.) The exploit involves a type of DCC command which can be sent to any person or channel, no matter what your DCC options are. (DCC is what you use to trade files, or to do private chat independent of IRC servers.) The author of mIRC has been notified, presumably an official fix will be forthcoming "soon" but that is his call. In the meantime, please don't clamor for more information, everything we are at liberty to say is here.
For now, the only known fix is to ignore all DCC requests entirely. Just type the following command, on a new line by itself in any window, beginning with the slash character:
/ignore -wd *
[Taken from: http://www.irchelp.org/irchelp/mirc/exploit.html]
BTW: This isn't a 'joke' of any kind... I myself experienced crashes of mIRC, and I use 6.11. The only solution is to ignore DCC chats or sends, with the /ignore command above.