When an item is thrown at, or moved to by any other means, the open quarter-block in a stair it is shot away from the block.

If you throw a block or an item at the open quarter-block in a stair, it is spit away from the block back at you, rather than just stopping next to it or going inside the quarter block. I believe this is a bug due to the new update that spits items out of transparent blocks (Glass, Glowstone, Half-Slabs, Stairs, etc.).