The clock as shown on the Minecraft Wiki (link below) isn't working. Instead of rapidly switching between stone and redstone blocks, it only replaces the redstone blocks with stone. This issue matters more than others because many redstoners depend on this type of clock.
http://minecraft.gamepedia.com/Clock_circuit#Setblock_clock
https://www.youtube.com/watch?v=6gWg3t7a1wY