If an entity is not in the lower half of a block space, attempting to place an upper slab or upper trapdoor in this space will succeed. After this, the entity will be shifted upwards or to the side as if a block were placed on them.

Steps to Reproduce

1. Obtain 3 full blocks and 2 slabs (the 2 slabs are the most important)
2. Place a slab
3. Walk on it
4. Place 3 blocks diagonally from the block space where the slab is (say, left 1 block and forwards 1 block as shown in the video)
5. Stand as close to the edge of the slab as possible, without falling off of the slab
6. Attempt to place an upper slab on the bottom most block on the pillar

Expected Results:

Nothing happens.

Observed Results:

A slab is placed, shifting the player upwards.

Video Example (unlisted)

https://youtu.be/FjEXaCnG1q4