When placing stairs upside down on the ceiling above me, in a 3x3 tunnel, so that the stairs I'm placing are directly above Steve's head, if I hold the mouse button to continually place stairs and move slowly to the side, a new stair will place before I've cleared the old stair. The animation fires but the stair does not appear until I've moved out of the old block. Then, the stair appears where my head was. Essentially, I place a stair in the block my head is occupying while my head is still occupying the space, but it doesn't appear until my head moves out of that space. Video of the bug in action attached.