Players take fall damage on slime blocks when sneaking in certain cases

The bug

In 1.21.2, a change has been introduced where slime blocks cancel fall damage while sneaking. (MC-275834)

However, players will still take fall damage in certain circumstances, probably when they have enough horizontal speed and start sneaking at a precise moment. This unintended behavior may potentially affect parkour to some extent.

Steps to reproduce

1. Place some slime blocks and build a 15-block-high pillar (shown in setup.png)

2. Stand on the pillar and switch to survival/adventure mode

3. Sprint and jump off the pillar, towards the slime blocks

4. Press sneak key exactly 1 tick before hitting the slime block

Expected result

You should not take fall damage while sneaking.

Actual result

You take fall damage.

Further details

• Upon taking damage, other things happen:

  1. You get teleported back to the position of the previous tick, and your momentum is reset.

  2. The following warning message is printed to log:
     [13:30:35] [Server thread/WARN]: <player> moved wrongly!
     This suggests that the issue may be related to the game's movement checking logic.

• The height of the pillar is not arbitrary. To effectively trigger this issue, the player's position on the last tick before landing must be more than 0.6 blocks away from the slime block.