Another instance of this bug in 13w01b. Everything needed to reproduce is visible. Piston's glitch when F=0 & F=2.
*Note: The top block must be powered via lever.
Note: the redstone dust below is required for the bug, and must be un-powered at the same instant the pistons are powered.
Made a short video elaborating on the behavior of pistons' apparent ability to duplicate when facing certain directions. Includes the phenomena of stacking "phantom" pistons. (Note, the "phantoms" will no longer be created once the twelve block piston-pushing-limit has been reached.
http://www.youtube.com/watch?v=_hQOf0LQoP0&feature=youtu.be