If you place an Hopper at the side of a normal block and under this hopper an other Hopper. Items or blocks from the first hopper can't pass through the hopper in the other hopper. But when you place an stack of an block or item in the upper hopper it passes this hopper and goes in the under hopper. And the upper hopper has no Redstone Signal on it!

This happens in Single and Multiplayer.