Just curious now, are there any real (and abused?) exploits in mIRC v6.16?
Yes. It's related to the auto join on invite option. There's a discussion about it on mircscripts.org
here, as well as an addon to fix the bug.
The bug was discovered a few months back (around June) by some of the ops in #mIRC on Undernet as far as I know (that's when I first heard about it, anyway), but it has been fairly widespread for a few months.
If you require bugfree code to call a program stable, there is no stable software out there.
I don't ask for bugfree code because, afterall, Khaled's only human, but I don't think it's too much to ask that an exploit be fixed.
mIRC has been the most stable program I've used over the years.
Notepad for me

A simple ftp client uses almost twice as much memory just after opening without connecting to anywhere as mIRC uses after a week being on multiple servers.
Tell me the same thing again after you've done a lot of copying (hilighting text) throughout the time that mIRC's open.