Minecraft wiki entry for Development_versions, associated text: MC-177 – [12w26a] Clocked piston arm turns invisible when exiting game/dying/etc.
MC-184 – [Beta 1.2] (Powered) redstone doesn't update right.
MC-5621 – [Beta 1.7] Blocks like redstone dust, repeaters, doors, and levers don't drop when the underlying block is pushed by a piston.