While testing out some of the new features in 14w18a, I noticed that this was happening. If this is a duplicate, I would like to say that I searched "14w18a sideways blocks." This obviously is a rendering bug due to the side texture of sandstone becoming the top texture. It was kind of difficult to show this, but I tried as best I could. If any additional details are wanted, then please say so, and I will update this.