When opening or closing doors (i've only seen with acacia) occasionally the top/bottom of the door will open only. So if you click the top half, only the top will open, and vice versa. I have repeated it many times while facing west and while flying towards the door while clicking. Sometimes the door seems to "duplicate" itself, with it looking like it is open and closed, all in the same block. You are able to walk through the half which "doesnt catch up"

http://i.imgur.com/bXiCNjC.png