When standing on mud (as well as many blocks that don't have a full block height) in water, aiming at a block on land, and walking slowly towards it off the side of the block, the player will automatically jump even if auto-jump is not enabled.

Left to Right: Mud (jumped), Chest (jumped), Cake (jumped), Slab (jumped), Path block (no jump), Full block (no jump), Sea Pickle (no jump)