/Fill and /Setblock 20hz Clocks "Jamming"

The /setblock and /fill Command Block clocks can now randomly cease by themselves. This happens on servers and even in single player.

The common setup for these clocks would be similar to :

Top Command Block : "/setblock ~ ~-1 ~ minecraft:redstone_block 0"
Middle Block : Redstone Block (being rapidly replaced)
Bottom Command Block : "/setblock ~ ~1 ~ minecraft:stone 0"

One of the easiest ways to reproduce is to create a long line of Command Blocks that set the next block along as a Redstone Block, which then triggers another identical Command Block and so on. It should look as though a single Redstone Block is moving along a line, being replaced by an alternate block behind it.
Much like this; http://gfycat.com/LightheartedEvenDwarfmongoose.

Set this up to loop around and then exit the map in single player. Upon opening the map again, often the Redstone Block has duplicated, and there are now two in the setup. Repeating this will add more Redstone Blocks until the entire system has "jammed" completely.

As far as when this behaviour began, the earliest snapshot I'm aware of is 14w33c, though it was likely occurring much earlier. This bug poses a threat to a lot of custom maps that utilise 20hz clocks, or any /fill or set block clocks for that matter. Eventually, all clocks in the map will cease to function. Besides good old fashioned Redstone Repeaters, this is the only clock type remaining that functions outside the World Border.