I chiseled some cobblestone in survival and i got this type of bits that are not same as cobblestone bits but when i place them they work same as cobble.

playing in 1.8.9 version 8.2

http://imgur.com/QPOQs8g

Reported by Zedner - http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2576750-chisels-bits-flat-colored-blocks?comment=197

Those bits are from Double Slabs, which shouldn't be able to be chiseled.