I have this world that I have used to test the 1.8 snapshots. In this world there is a village that I do my testing in. To day I went in the world and the stairs had a strange light bug on them.
I expected the stairs to have the same look as they always have had, but they had a strange dark spot on them.
To re-create the bug just find a village in a world and take a look at the stairs on top of the houses.
I have seen that too. The stairs look different depending on whether there is a block below them or not, or so it seems.