A couple days ago I noticed one tile in my screen was grayed out (sort of).
My first thought was that the app was updating but as soon as I went to the Store there are nothing downloading or just updated.
In this picture you can see Whatsapp darker than the others. I couldn't open it for about a minute.
After that Tweetium (the tile above Whatsapp) did the same for a minute. Again I couldn't open the app.
I asked the official Windows Phone Twitter account and they couldn't help too.