Stair particles are a bit odd. They will come out of (what seems like) the wrong part of the stair.
What I expected to happen: Particles on stairs acting similar to regular blocks.
What actually happened: Particles on stairs not acting similar to regular blocks.
Here is a video if my explanation was does not suffice:
http://www.youtube.com/watch?v=OfQa3UUfZ2A&feature=youtube_gdata
I apologize if this is a duplicate. I could not find another bug report that brought attention to this bug.
I can confirm this. It's a bit odd. A little graphical glitch. Not important but you are right, this looks like a bug.