If you hit a mob at a specific position in front of a specific wall of blocks, it falls through the ground where it's standing.
See the pattern with the images attached; to reproduce it, place a 3 block high wall, with a two block gap in the middle. If a mob is spawned one block beyond the wall and is hit, it will overpass the ground. If you place the mob inside the gap it won't work. If the mob is 1 block to the right or to the left of the gap, it won't work either
It works with 2 block high mobs, and also blazes and rabbits. It doesn't work with any other animal or mob. It works also with slabs, sand, furnaces and any block I tried as ground. It works if beneath that ground there's a gap or empty space, like in a house with two floors.