I use the chat dccserver feature extensively... and I noticed random crashes of mIRC when people disconnect from this server (ie, when the dcc chat window is closed).
It's difficult to reproduce it, as it appears to be random, but it always happen when closing the dcc window.
I've written a script which crashes mIRC with a GPF, tested under all versions from 6.0 to 6.16, which is not exactly with the chat dccserver, but I think this is the same bug, still.

Code:
ALIAS testcrash {
  var %i = 1
  while (%i <= 20) { dcc chat $+(127.,$rand(0,255),.,$rand(0,255),.,$rand(0,255),:,$rand(1,65535)) | inc %i }
}
ON *:CLOSE:=:window -c = $+ $target


This basically opens 20 dcc chat windows (with a loopback IP... it won't connect of course, but it doesn't mind), and then closes all dcc windows. mIRC will crash before all windows are closed.

Of course this test is a bit "brutal", but this is the only way I've found to reproduce the random crash. (which can happen if there are only 2 dcc open !).

mIRC 6.0 => 6.16 / Windows 2000 Pro