I don't think this particularly qualifies as a bug, but I ran across this with an unknowing user.
When you add nicks to the highlight list and separate them with commas, be sure that you don't inadvertently type 2 commas. I had a situation with a user that had accidently typed 2 commas in the middle without a nick inbetween. This would cause his mirc to completely lockup when joining a particular channel. Perhaps the next version could include some sort of failsafe for that.