[CLIENT] Alpha Client Visual Bug (Alt + TAB does a White Rectangle)

I decided to launch a Game in the Alpha client, i watched a video since my pick but when i came back on the game, there was just a white rectangle instead of the game, i had to quit the game and now i get 15 minutes of waiting. WTF ?

How it happened :

1)Get in Bans and Pick 2)Alt + TAB to another window 3)Alt + TAB to the LoL window again

Maybe this will not happen to you, but yet it happened to me.