The player and other entities automatically jump when falling into 1-deep water from certain heights. This was first introduced in 18w07a, which introduced hacked water (it does not happen in 18w06a).

The water must be 1 block deep; it doesn't happen for deeper water.

This happens with a 1-block drop (from the orange level) and a 6-block drop (from the pink level) into still water. It also happens with drops of the same heights into flowing water (level 7). However, note that the 1-block drop into still water also triggers MC-135212, so it is not recommended for testing.

For the flowing water variation, no buttons need to be held, as the water pushing against the wall is enough. For the other variations, you must hold forward.

Armor stands are affected if given force (/execute as @e[type=armor_stand] store success entity @s Motion[2] double .1 if entity @s), and also can be reproduced with the flowing water variation by pushing yourself into the very corner and then running /summon armor_stand ~ ~ ~.05 to summon an armor stand near enough to the wall. This does not seem to happen with lava.

To reproduce

1. Download water_fall_auto_jump.nbt
2. Load it into the world
3. Fly up to the pink layer, on the center
4. While looking very slightly to the right, hold forward so that you slowly move towards the edge
5. While falling, still hold forward
6. When you enter the water, you will automatically jump once
7. Attempting to reproduce from this height with the 2-deep setups on the left will not work (some of the other heights will still trigger, but that is due to MC-135212)