My model has a pair of objects, one a mirror image of the other, like bookends or a pair of wheels. They use the same cycles nodes material involving a normal map. Ridges on the one object appear as grooves in the other. It looks as if the normal-x component and the mesh x coordinates aren't in sync.
Note that I prefer to use normals, rather than the Displacement input of the Material Output, due the nature of the work, fine details. I don't care about seeing bumps or groove profiles along the edges of the objects. But I did try Displacement anyway, and got the same result. The attached images might not be the perfect example, but shows the problem.
I thought maybe "Apply Transform" might fix it, so each object has points in the same world coordinates, but nope, this didn't fix it.
Added note: experimenting on a new scene, it seems that applying the transform causes the problem. For the example shown, I had already applied the transform a few weeks ago when starting the project. So the question is: how to have ridges not grooves for an already-applied-transform mirrored object?