Whenever you press a key on your keyboard while you are in the channels list window, mIRC will search for a channel that starts with that character, and if you keep pressing it, mIRC will select the following channel that starts with that character, and so on; but if you pressed an alphabet key and the channel starts with that letter in uppercase, mIRC will change that letter in the name of the channel to its lowercase equivalent.

Let's say that there's a channel named #MIRC; when mIRC highlights this channel after you pressed the key "M" in your keyboard, mIRC will also change the name of the channel to #mIRC in the channels list window.

Maybe I'm missing the point, but M = m in channels, no?
The IRC server will recognise "#mirc" as #MIRC, #mIRC, #MIRc, #miRC, #miRc and so on.. it's all the same channel..

Maybe I'm just missing the point..

correct, /join #Mirc is the same /join #mIRC or #mirc or #MIRC, but he is talking about mIRC changing the case of some characters in the channel list window.

yes, it happens here too

i would call this more of an easter egg than a bug

Solved in mIRC 6.15
37.Fixed channels list lower-case display bug on keypress.