Dropper towers (designed to work as an item elevator) seem to be very tempermental. The same design works flawlessly in 9 out 10 instances, but it seems that depending on where the tower is located it may not work, even if no other blocks are touching it.

The item elevator in the image is bugged, but all the other ones behind work just fine. It seems to me like the half-slabs are being recognized as blocks; even though the redstone on top of them looks like it recieves the pulse from the comparator, the signal does not travel to the top and the items get stuck in the 4th dispenser.

I'm not sure if this has to do with the droppers, or the half-slab ladders leading to it. I've tried unsuccesfully to recreate the bug in creative; whether or not the elevator works seems to be dependent on where in my world it is located.