raytrace shadow transparency
 

Check this out...

I have placed a shader on a plane. The object is to create a facade. I placed a colour .tif image on the colour channel and a black and white (no shades of gray) .tif image on the transparency channel. When I render this puppy, I can see the faint outline of the plane on the floor. Why is this happening and what is the solution? I was always under the impression that black is black and white is white....aughh!

Is your raytrace shadow depth 3 or more?
(in your light settings)

That cured mine in the past:)