Steps to reproduce: Make sand or gravel fall on bottom slab. Repeat and/or try to mine it (shovel or hands).

EDIT: There must be air below the slab.

Affects trapdoors as well (found by Skaijie)

Original report: There are actually four blocks (two sand blocks, two gravel blocks) on the picture. I did try to mine them (using shovel or bare hands) and it is damaging the tool but not breaking the block. I did save the game and reloaded and those were still there (originally only with sand, I tested gravel as well when I found this). They break eventually, sometimes after few seconds, sometimes after quite a lot of time.