Since the players height changes when sneaking, i decided to experiment
What I expected to happen was...:
Be able to walk on a trapdoor thats on the floor, while a trapdoor is also on the ceiling.
What actually happened was...:
unable to walk in the gap (even while sneaking)
Note: tried with trapdoor on ceiling+ 2 layers of snow (which actually gives less player space) but im able to sneak onto that
Steps to Reproduce:
make a 2 high block
Place trapdoors on the ceiling and floor. then sneak to 'try' to walk over
Note for picture: http://imgur.com/SvponSu
Left side, trapdoor + 1 layer of snow
middle 2 trap doors
right side: trapdoor + 2 layers of snow for size comparison.
Two layers of snow looks higher than the trapdoor, but you sink into the snow a bit, making it a bit lower. One layer of snow is the same as nothing at all as you sink into it completely. WAI.