The player gets completely stuck - cant move, jump, crouch or turn around. You can only break the block in front. Reloading the game doesnt help. Breaking the culprit trapdoor frees the player, but can only do that if you freeze with your cross-hair pointing at it.

I was able to recreate it while standing at the locations on the attached image. Just stand on the trapdoor and open/close it a few times and wiggle until it freezes the player. Maybe it help to have another trapdoor adjacent to the one being opened.