Whenever I place inverted steps on the inside of a corner there is a shadow that will never go away. If additional steps are placed next to this step the shadow may stretch down along the newly placed steps or it might not. It does not matter the lighting in the area, the corner of the step will always be blacked out. I do not recall if I have seen this on upright steps. Attached is a screenshot of the phenomenon. The first one shows the shadow stretching along the ceiling, and around the corner (even along straight steps). The second one shows it contained only to one block. The presence of torches makes no difference in the lighting level.