Doors and trapdoors have a convenient grid of four 'windows' in them. Despite that these things are 3 'pixels' thick, looking at the outside edge of them, there is no 3-pixel-thick border to the windows, when you look through them.

This might be intended for better visibility, now that I think about it, but I thought I'd bring it up, since it seemed like I shouldn't be able to see straight through a trapdoor window, to see the cobblestone that it's attached to.

It immediately reminded me of MC-102 and MC-3908, but on the other hand, you don't have to put a block next to it for it to occur; it occurs pretty much no matter what is next to the door / trapdoor.