When a sticky piston (facing up with a block on it) is retracted while an entity is standing on it, the entity appears to sink into the ground.

1) Place a sticky piston facing up.
2) Place a block on sticky piston.
3) Power the sticky piston
4) Stand on the sticky piston
5) Unpower the sticky piston

The player will then momentarily appear to sink into the block.
Note that this doesn't happen if a block is not attached to the piston.
This also happens if slime blocks are used.

[Mojang] Panda comments:
" I think this is not just for a piston retracting down, but also for ones in negative X and negative Z: https://youtu.be/tuPTiP09RBg "