The bug

If a player is standing inside of or on top of a block which slows movement, and a horizontal piston pushes the player's top half, inconsistency can be observed depending on the slowing block:

• For water, lava, soul sand and honey blocks, the player is pushed the whole way over by the piston, as if there was no slowing block.
• For cobwebs and sweet berry bushes, the player is only pushed slightly and enters crawling mode.

Expected results

Either all of these would result in the player being pushed all the way (preferred), or all of these would put the player into crawling mode.