When I set up a horizontal /fill clock (The Westward command block having the command "/fill ~1 ~ ~ ~9 ~ ~ stone" and Eastward command block "/fill ~-9 ~ ~ ~-1 ~ ~ redstone_block") it immediately ends up stone, and the Eastward command block has as last output "8 blocks filled" instead of 9.
In his VillageBuilder, sethbling has a working horizontal /fill clock exactly like this, but even on his world when I try making one more by copying and pasting the blocks it doesn't work !