Video: http://youtu.be/jz7PSGaT3CM

When you have an horizontal hopper loop with multiple hoppers locked, something like this:

---------!
!---------

Being the - and ! hoppers feeding into each other in a loop.
When you have 1 item in the loop and send a pulse to unlock all the hoppers at the same time so the item will go to the next hopper in the loop, that case works fine... something like this:

---A---!
!----------

Being A the item in the hopper, when unlock the item will pass to the next hopper correctly.

If you have multiple items in the loop, one behind the other, like this:

--A B--!
!---------

When the pulse is send to the loop to unlock it, 3 things can happen depending in the position of the items in the loop (this is consistent, always happen the same in the same spot):

Case 1:
--AB--!
!---------

---AB-!
!---------

Case 2:

--AB--!
!---------

---C--!
!---------

C = A+B

Case 3:

--AB--!
!---------

---A-B--!
!---------

Only the first case should happen, Item A in the Hopper 1 should go to the Hopper 2 and Item B in the Hopper 2 should go to the Hopper 3. This problem is positional, the result may change depending the position of the hoppers in the map and the lenght of the pulse.