Once, a Hopper connected to the top of a Furnace transformed into a Furnace while I was away. It had the side texture of a Furnace, but was facing the wall, so I couldn't tell exactly what it looked like.
When I right-clicked on the bugged Hopper, Minecraft crashed. It's still there after reopening Minecraft.
Somehow, MCEdit also draws it as a furnace. The tags for it being a Hopper have been deleted, although it is still recognized as one.
Breaking it also caused a crash, but did remove the Hopper.
Crash might be related to the hopper's Inventory not existing.
I'm fairly certain only completely breaking it causes a crash, couldn't test since hopper was broken in creative mode. Unable to replicate.