Mac OS X 10.6.8
When multiple hoppers are stacked on top of one another, and two or more of the same item are placed in the top hopper, the items are drawn into lower hoppers in "packets" of two items per update.
ATTEMPTS TO SOLVE:
I tried using a comparator to sense when one block enters the top hopper, and then inverting the signal and shutting off the two hoppers beneath it, but this still transfers two blocks rather than one.
ATTACHED IMAGE SETUP INFORMATION:
In the attached image, the top hopper is empty, the second hopper has 22 iron blocks in it, the third hopper is full with iron blocks, and the fourth and fifth hoppers are empty.
SECOND RELATED BUG WITH THE SAME SETUP:
If a non-iron block is placed in any slot in the top dispenser, and then iron blocks are placed in a slot to the right of the non-iron block, then only one iron block is drawn at a time.