Copy anything to the clipboard with command-C -- OR bookmark your page with command-D -- and then immediately press command-W to close the window... Opera will Quit and then relaunch all windows.
This only happens if there are no tabs in the window... that is, if command-W fully closes a window, and does not merely close the active tab bringing you to another tab in the same window.
Can anyone else duplicate this, so I can be sure it has nothing to do with an extension, etc.?