If jumping into a pool of water one block deep against a wall, the player will occasionally bounce back up a block. This is a seamless animation (similar to the animation when bouncing on a slime block), so I don't believe this is a y-coordinate glitch.

It happens both with and without the spacebar pressed (my first thought was the keypress interval syncing with the moment I hit the block under the water) and seems to occur more consistently when jumping into corners.

I've never noticed this previously, though I don't know if this is specific to this snapshot or not.

Easiest way I have been able to reproduce is the setup in the attached screenshot. Jump from one side to the other and hit the wall on the other side, falling into the water. The player bounces back up before falling into the water as is expected. It will happen both when sprinting and when walking (to test walking, just reduce the river to 2 blocks wide).

Best guess is the player is landing between the block above and the block below the water line and is pushed out.

18w21a Video: https://www.youtube.com/watch?v=biVtcmHmMm4