made a video of it, it is very difficult to explain -> http://www.youtube.com/watch?v=qXVIV1fNWjI
Edit:
Not so difficult to explain:
Blocks like Redstone dust, Repeaters, Doors, Levers do not drop when the underlying block is pushed by a piston
Related issues
relates to
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Confirmed, but is this a bug or Working as Intended ?
Fixed as of 13w01b.
Related to MC-3287.