Minecraft 19w02a, Minecraft 19w03a, Minecraft 19w03b, Minecraft 19w03c
When you attempt to place scaffolding in the same block as yourself, you can't place it.
This is inconsistent with other blocks that aren't solid, as you can place those within your own hitbox.
Signs can be placed within yourself, as you can walk through them.
Scaffolding can also be walked through easily, as it is not a solid block, but cannot be placed within your own hitbox.