i was making a sugarcane farm with hoppers. I had it all worked out and everything. Then I noticed that my chest was being filled with more sugarcane than would have been farmed. I tried it with any item and it works. Toss an item at the edge of the hopper. It will stay there but the hopper will continue to grab it but not delete the actual item thus duplicating the item. Here is a link to a youtube video I recorded of the bug happening.
http://youtu.be/Efilr13-FIg - give the video a few minutes to finish uploading...my upload internet speed is horrible. It's only a 30sec video but shows exactly what is happening.