Due to the new "natural texture" mechanics in 1.8 that does the textures of some blocks rotate randomly (like grass and TNT), when a block is moved by a piston, its texture rotates. Probably because the block is different when it's being moved by the piston, and then when the game sets the block again, it has another texture as if it was placed randomly in the ground.

It looks like the texture rotation is preset, in a way that if you put a block in a certain spot, this block will always have the same texture rotation, but this causes weird problems when blocks are moved from one spot to another. Because of that, I don't know if this will get a "Works As Intended" resolution, or a "Confirmed" status.

I didn't find any tickets reporting this exact problem with the words texture, piston, block and rotate.

Steps to reproduce:
1- Do a setup with a piston moving any block with a "rotatable" texture (TNT can be clearly seen).
2- Trigger the piston.
3- The texture of the block rotates, as if it was randomly placed in the ground.