This sounds like the mIRC lock dialog, I experience similar corruption using Compiz when switching back and forth from mIRC.
I'm guessing mIRC thought it was being minimized to tray when you switched workspaces, as other applications I've used have falsely activated minimize to tray functions also. I don't use the Lock feature myself, so I can't confirm this for you. Just double check your Lock settings (in the Lock section of Settings) and tinker as you wish.
