Simply, when I was trying to place a block on the block I was looking at, my crosshair clipped through that stair and let me select the block adjacent to it.
For more info, view the photo.
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Cannot confirm, see screenshot.

Can reproduce in 1.5.2. However, it depends on which way the stairs the crosshair aims at is placed. The raytracing is done with the "real" stairs, not the visuals of the stairs. The aimed stairs block can be placed in two orientations which both produce the same visual result.
I'll attach 'stairs_orientations.png' where the upper placement (after adding a stairs block to the corner) shows the issue, while the lower placement will not.
EDIT: First I thought this is a minor bug, but then I realized that this can be used to shoot arrows through a visually solid stairs (near the corner). So, I think the real bug here is that the stair's raytracer method uses only the block's data to figure the shape of the block, instead of the visual shape calculated with also the neighbor block data.
Duplicate of MC-1307.