I'm not sure whether this is a serious bug, but I've never seen it before. Unfortunately I don't know exactly what happened. I joined an all random game on the Proving Grounds, and when the LoL logo popped up before the loading screen, there was a very small window behind it. I clicked on the little bit of the window I could see, and both the logo and the window disappeared. The loading screen didn't come up, and when I restarted the client, it kept crashing after I clicked "play" on the launcher. I wasn't able to reconnect until I used Windows Task Manager to terminate the existing LoL processes.

Novas Necros I see that little window too, but it doesn't cause me any problems. The game didn't launch properly but the proccess was still open when you tried to relaunch it, this is why you had to use the task manager in order to make it work. If you have the problem again (game not launching) try to repair the installation.