The bug

When the player is in a space that is just one block tall, it can walk and sprint at normal speed, which seems illogical since that is not possible if the player is sneaking in a 1.5 blocks tall space (MC-147270)

To reproduce

1. Put a trapdoor at a wall and open it
2. Go inside of the trapdoor
3. Close the trapdoor while you're standing in it
4. You're now "crawling"
5. Dig a tunnel
6. Walk into that tunnel, and try to sprint
7. You move at normal walking / sprinting speed

Video

Minecraft 1.14 Pre-Release 1 2019-04-10 17_26_56_Trim.mp4