It's not a bug, it's a feature.

So, I'm re-arranging my masteries to better suit my current champion when my mastery page goes blank and wont close out. So as the clock slowly ticks away and I grow more and more concerned about whether or not my masteries, summoner spells, and runes are correct for my champion I am all the while comforted by a tannish-yellow screen blocking my view of everything important.

The link is a screencap of the bug I was experiencing. It doesn't get in the way of the actual game once the match starts, thank god, but it's still super annoying and could end poorly if you don't have your rune page/mastery/champ selected/ etc etc.

So uhh, fix it. I guess.