When a corner-stair is placed underwater, or adjacent to water, the flat back of the stair does not display correctly. It displays the "waterfall" effect that would be seen if an airspace was there. Previously, it displayed the same as a full-block.

I will post images of this later today

This can be demonstrated by building a wall out of full blocks, then replacing some blocks with two stairs interacting so one is a corner-stair. This will make a wall that has the stairs visible on one side, and will appear to be a flat wall on the other side. If you then place water source blocks against the side of the top of the smooth side of the wall, the corner stairs will appear "highlighted" by the apparent water surface that is rendered.

I have not yet tested if it also does this with regular stairs.

This did not happen in 1.7.2 I am unsure if it happened in 1.7.4, but I don't believe so.

I believe that the render engine is having the water treat the entire corner stair as an airspace.