What I expected to happen was...:
I expect the trapdoor to close on the player like usual.

What actually happened was...:
When closing a top half trapdoor that would collide with the player at half height, the player assumes the swimming animation and is pushed down to one block tall, under the trapdoor.

Steps to Reproduce:
1. Place trap door on top half of block at players waist
2. Open trapdoor
3. Step into the area of the trapdoor
4. Close trapdoor