When placing stairs, the way it draws the texture is wrong. This is more evident on corner stairs.
The top half of the texture using normal stairs and the bottom half using upsidedown stairs, have the texture shifted out of phase with the other half by half a block, with gives a very unpesent line in the center of the block and means the texture does not match the block it is supposed to be a stair of.
In the attatched picture the highlighted block is a corner stair with the top half of the texture shifted.