I have only tested this on the Dark oak door.
When the door is placed on grass (facing outwards) and there are some zombies outside. if you open said door and hit them while there in the door block (In used Iron Sword) They Sink 1 Block down like the block is not even there
(Note: there was no special block under the grass. it was dirt when i 1st found it and sandstone when i confirmed it.)