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After the LoL client launches (not the patch prewindow) an empty space gets put into expose. If you go to the all window mode there will be a large blank spot that you can mouse over and it will turn blue (just like a regular app), but when clicked on it sends you to the LoL app, but with no actual window. Sometimes it recognizes that there is an app running (but still no window shows) and sometimes after clicking on it it just goes back to the app you were previously on.
Hopefully this makes sense, not the easiest thing to describe.