I have found two issues with the "Limit channels to:" option under lock in the options dialog. However, while this is technically a bug, I strongly doubt it would ever happen in normal use, so it is not something urgent. To replicate, first make sure you are in a channel. Next, in the options under +other -> lock, check the "Limit channels to:" box and make sure the channel that you are in is not listed. Click ok.
Next, do /hop -c in that channel. One of two things will happen. If you have "keep channels open" checked in the options, you should see something like:
* Attempting to rejoin channel #somechan
* Topic is 'something'
* Set by someone!etc@blah.blah.etc
with an error message in the status window saying
* Channels locked in options dialog
mIRC will automatically part the chan, but the channel window stays open.
Now, if you go back and uncheck the option for limiting chans, and try rejoining the channel, it will not let you. You get no error message. To rejoin, you have to close the channel window first.
Now, if you have "keep channels open" unchecked in the options, it will close the channel, as expected, when you try to cycle. However, when you unlock the channels again, mIRC will let you /join the channel, as I could plainly see in a /debug window, but the channel window does not open. To get it to open, you have to disconnect and reconnect, then join.
Again, not an important bug, considering that it probably would never happen in real life, unless someone wants to be in a channel without the window working, but I still decided to pass it along.
Oh, yeah.. mIRC 6.15 on winXP