When standing under a 2 block high ceiling, mobs you spawn that are 2 blocks high, will spawn inside the block above you instead of at ground level with player. At this point mobs fall through the block above the player but don't get stuck. (See 2nd photo)

I tested this further by making a room that has a 2 block high ceiling. The room is 4x5 blocks in length. (See 1st photo) Spawning zombies in this room will sometimes spawn them in the walls, since they're always being spawned above the player.