When a platform of solid blocks has scaffolding as one of the platform blocks (so the top of the scaffolding is flush with the platform), flowing water prevents the player from starting to descend the scaffolding. To recreate, make a platform with water flowing across it. Replace a platform block with scaffolding until the scaffolding reaches the ground and the water flows across the top of the scaffolding. Try to enter the scaffolding by crouching/sneaking on top of the scaffolding.