If there are two blocks placed diagonally touching and vines are placed in the non-block occupying spaces the hitbox is treated as a full block. Refer to screenshot to see what I mean.
I'm very sorry if this is not a bug, just say that in the comments then the next week or so delete this.
The blank texture is my texture pack's stone texture. I did it to avoid having distracting surroundings.
I have more if you need.
CREATE WORLD IN 1.6.2 (default) and go to coords in picture.
It is only naturally spawned. Untested in new generation.