When you close a door while standing within the hitbox of a door, you start swimming beneath it. happens when flying, and when there is no block beneath the door. 

What I expected to happen was...:
I expected to stand

What actually happened was...:
I swam

Steps to Reproduce:
1 find or place a door in snapshot 19w14a

2 open the door and stand where it will close

3 close the door, and now you are stationary swimming on the ground.