Blocks with inventories cannot be placed next to/on each other. Chests cannot be placed on other chests or next to each other above ground level. Multiple hoppers cannot be placed in a row unless done at ground level, and if this is done they don't point into each other. Hoppers cannot be placed above droppers or other hoppers without the aid of extra solid blocks being used to place the hopper against. Items will not move from one hopper to the next in a row where they should point to each other because they don't. In gameplay I've watched from other users they are able to perform these actions.