Placing two slab blocks together normally results in the appearance of a single full block that drops the two slabs when broken. As of 14w17a, all slabs placed this way, with the exception of wooden slabs, result in a block that is generic in appearance. (See 1st image; I am not familiar with this type of block and could not find it in creative or the wiki)

Note that the double slab still behaves correctly other than its appearance. Breaking it will still drop the two slabs you originally placed.

Wooden slabs are unaffected by the bug. I've confirmed the bug was not present in the previous snapshot, 14w11b.

The images below show a single slab, then the corresponding double slab for every slab type in the game. The first image is from 14w17a, the second is 14w11b.