Nothing major, just some slight 'annoyances':
compare mIRCs display for /userhost NoSuchNick and /ison NoSuchNick:
userhost: no such user
: no such user
mIRC doesn't attempt to resolve 'localhost' if it receives it from a userhost when attempting to find my local info. Maybe mIRC should use the /localinfo -h option when a userhost reply returns localhost?
When reconnecting to a server/network, mIRC will often send multiple 'JOIN' commands for the same channel. Not too sure why as I only have one 'rejoin' type option on (rejoin on connect) and 'keep channels open', and there are no scripts sending the command.
Lastly this one is hard to explain and happens infrequently but often enough to become a pain, mIRC will often 'pause' from displaying data to a window until that window is clicked on (usually the status window), the scrollbar will update to reflect the extra lines of data, but mIRC won't actually show the text until i click the window to make it active. This occurs a lot with multilined output such as /whois, /lusers, /motd etc.