When an item falls (or is dropped) on a stair block it can bounce. I have found that when it is over the part of the stair block that is shown in the file below (The part of the block.png). I tested it with creative and survival and it works in both. Also, works with either Fancy or Fast graphics. It does no harm to the item, player, or game. It is just a weird bouncing item graphical bug.
I have not tested it in MineCraft 1.6.1 or lower. Fin.