On Freenode, I am able to trigger this bug by setting the following modes in order, using distinct MODE commands, i.e.:
- /mode ##mIRC_bug +k test
- /mode ##mIRC_bug -k *
- /mode ##mIRC_bug +k *
mIRC now (incorrectly) thinks that the key is
test, according to the "mIRC Channel Central" dialog you see here:
* Sebastian sets mode: +k test
* Sebastian sets mode: -k *
* Sebastian sets mode: +k *

Incorrect display was confirmed by using a second connection to attempt to join the channel while the key was set. I used the following commands:
- /join ##mIRC_bug test
- /join ##mIRC_bug *
Only one of those commands worked, and this is how I confirmed that mIRC is displaying the wrong key in this dialog.