There is an interesting discussion in the SL Forum about a problem some people have had making Normal Maps for Second Life™. Jake Koronikov opened the thread. Jake had a problem with the normal maps creating a visible seam in the model.

For instance, the seam between the front and back sides of the avatar leg. Of you were making a manikin and using a normal map on it that seem would become visible as a step on the surface rather than the smooth surface it is supposed to be. You can see his images of the problem in the thread: Tangent spaced Normal Map seam visible in UV border area. (2017 – The images are gone.)