Ok, to start out I'm just going to explain everything as best as I can..
I always have my mIRC client maximized, along with all of the windows maximized.
This is the second time this bug has occurred, I do not know what I was doing the first time it occurred, so I'm just going to give all of the information I possibly can.
A friend of mine had signed onto irc with his mobile irc client, so I decided to bug him a bit, I ctcp'd him, notice'd him, notice'd the channel, attempted to dcc him but the file didn't exist, msg'd the channel, talked a little bit (didn't switch any windows or anything) moved my mouse to the left (my mouse has tapping off) and boom...this happens.
http://img3.imageshack.us/img3/7121/mircbug.pnghttp://img522.imageshack.us/img522/6730/mircbug2.pnghow I temporarily fixed it: unmaximizing, and remaximizing the window.
It has not happened since then.