Using hoppers in a filter setup with a row of hoppers pointing to the right on top and a row pointing down underneath (as shown in screenshot). The bottom row of hoppers have items in their first slot so they pull that item from the hopper above as it is moving along.

This does not always work, in 13w02a and now more often in 13w02b the items moving through the top line of hoppers do not get pulled into the bottom row.

I believe the intended feature is that the bottom hopper will pull an item out of the one above if that item is in the first slot of the bottom hopper. As stated, this does not always occur.

Images are:
1. Setup of hoppers with top row pointing to right, bottom row points downwards

2. Inventory of third hopper along in the bottom row. Displays the setup to filter cactus from the hopper above.

3. Inventory of the hopper on the end of the line in the top row. Shows left over items that were not filtered out (bottom row has items to filter melon, mushroom, cactus and pumpkin)