You know how you can use 1-0 on the keyboard to view layers 1 through 10, and how you can use ALT-1 - ALT-0 to view 11-20? Yeah, you probably knew that already. BUT, you can use - and = to view layers 11 and 12 respectively. Okay, you may have known that, too. But, if you do ALT-- (That is, ALT-minus), then you will view a layer that can’t normally be accessed. It doesn’t show up in the layer buttons, but it functions the same as a normal layer. Don’t know if it was intentional or not, but I figured I would tell you guys.
Thats interesting but it doesn’t seem to render an object placed on it. It does however cast shadows on objects on other layers.
Possibly useful but it can be achieved in other ways. Probably just an undocumented feature which makes us love Blender even more.
“But they were all of them deceived, for another layer was made. In the land of Blendor, in the fires of Mount Bloom, the Dark Lord SaurTon forged in secret a master Layer, to control all others. And into this Layer he poured his cubes, his monkeys and his will to dominate all splines…”
BUT, you can use - and = to view layers 11 and 12 respectively. Okay, you may have known that, too. But, if you do ALT-- (That is, ALT-minus), then you will view a layer that can’t normally be accessed. It doesn’t show up in the layer buttons, but it functions the same as a normal layer. 
Don’t know if it was intentional or not, but I figured I would tell you guys.