Just throw an item onto the edge of the hopper, it will go through but will come out of the hopper as 2 if a stackable item. If A non-stackable item it will continue to fill the hopper and not stop.
Environment
Operating System: Mac OSX 10.8.2
Java:
java version "1.6.0_37"
Java(TM) SE Runtime Environment (build 1.6.0_37-b06-434-11M3909)
Java HotSpot(TM) 64-Bit Server VM (build 20.12-b01-434, mixed mode)
52nd duplicate of MC-5998, please use the search function to see if your bug has already been submitted. Currently over 40% of tickets are being closed as duplicate.