In a hopper array (like for an item sorter), a hopper beneath another hopper should take precedence in pulling an object from the upper hopper rather than the upper hopper pushing into the adjacent (same Z) hopper. With Spigot 1.8 and Spigot R0.1 SNAPSHOT b1660 not all items work this way. For example, cobblestone, dirt, and gravel seem to not get pulled into the lower hopper, but coal, andesite, and diorite work correctly. Switching to Minecraft Server 1.8.1 solved the problem, would much prefer a Spigot solution.