If i understand you correctly:
IF the nickname:
is in the nicklist, or in the channel,
and you type:
in the edit box, and send it to the channel, mIRC gets confused, and sends it to the Nicklist.
Well, it still makes no sense, but either way, it sounds like a script glitch, turn off all scripts and unload all DLL files,
and try your glitch again, if it still works, repost with better details, and correct spelling.