Any mob head/skull, even custom ones, can replace a non-solid block, entity or a block with a smaller hit box than a full block, when placed inside the occupying space. Those are buttons, lever, tripwire hook, trapdoor, doors, anvil, chests, fence/wall, fence gates, slabs, stairs, enchantment table, brewing stand, signs, ladder, flower pot, glass panes/iron bars (connected), etc. It can be done in all gamemodes. When a chest is replaced, all items (luckily) do drop out. This doesn't occur in 1.7.5.
Steps to reproduce:
-Place a fence or a lever on a ground or a wall.
-Grab a head and place it on the ground/wall where the object is placed.