Hello People of Minecraftia!
     Alot of SethBling's recent mapmaking tricks have involved redstone clocks that activate every tick. If you don't know how to make theese, it's basically just two command bolcks being powered by the redstone block, with one setting the power block to be a non-power source block, and the other immediately setting it back to a redstone block. the problem with these clocks, as with most, is that they can stop if there is a sudden chunk error or something.
     There are two clocks that are entirely constant, though: hopper clocks and CMD cart clocks. The maps I'm in the process of creating, though, require one-tick clocks for some of their functionality, so I use CMD cart clocks (powered command block minecarts with the command "/setblock <x> <y> <z> redstone_block 0 destroy") to keep them intact. I have a command block  powered by the one-tick clock constantly summoning FallingSand (which will dissapear immediately) next to another command block powered by the CMD cart clock with the command "/testfor @e[type=FallingSand,r=1]". Then I hook a comparator and a NOT-gate up to it, which powers a command block that sets the block powering the one-tick clock to a redstone block using the "destroy" argument.