I was playing Minecraft in creative mode, in the Nether, and tried to use the /fill command because I don't feel like placing 20 fence blocks manually. I checked to make sure the X and Y coordinates were the same, so it would be a simple, one-block wide and one-block long line. It instead made a one-block wide wall of fence blocks that extends (as far as I'm aware) to the very bottom of the Nether.