You really need to give more information before the bug can be verified.
The only issue I can think of would be how mIRC (and the ircd) handles bans prefixed with multiple :'s (eg: ::::::nick!user@host). This causes some confusion as mIRC/other ircds do not know whether the : has been used in transmission or is actually a part of the banmasks. It causes further confusion when the ircds 'strip' a colon as they distribute the mode change (meaning a ban set on one server will not match a ban on another server). The server developers can fix this by stripping all leading :'s from bans.
Please disregard if this is totally irrelevant.
Hope that helps