If you give yourself a speed 1 or speed 2 effect, and you try to sprint along a staircase made of slabs, it causes you to stop sprinting, as if you were trying to sprint up stairs, randomly.
Reproducing:
1 Enter a creative world
2 Construct a straight staircase increasing by 1/2 block up for every block horizonally, using slabs.
3 Get a speed 2 potion
4 Attempt to sprint up the staircase
5 Watch as your attempt fails after 2-10 steps