You Can Duplicate Dragon Eggs, Sand, and Gravel

Piston duplication bug.

Dragon Egg: http://www.youtube.com/watch?v=yv65iXhSQrQ
Sand/Gravel: http://www.youtube.com/watch?v=rFqVD53ppAU
(The above links are labeled 1.3, however they work in 1.4 last time I checked.)