Minecraft wiki entry for Redstone_components, associated text: When two redstone dust trails are placed vertically diagonally (one block over and one up, or one over and one down), the lower dust trail appears to crawl up the side of the higher block to join the other dust. This linking can be cut by an opaque block above the lower trail, which prevents the two trails from connecting. If the higher trail is on an upside-down slab or upside-down stairs, the higher trail configures itself to point toward the lower trail (and other adjacent dust), but the lower trail (although visually) does not configure itself to point toward the higher trail (including not appearing to crawl up the side of the slab or stairs).