Baked normals Look a Strange For a few months I'm researching and making little tests to better understing and optimize a LowRes Workflow, And this "error" continue to occurs on this particulary model that I'm working on, It was baked using tangent space, I tested in Substance Painter and Unity to see if It was a Blender Related Shader issue, and It was less predominant on the others aplication that a tested on.

But that was enough chit chat, here's a screen grab.

 

And thank you for reading! Have a nice day.
A likely scenario would be if the normals were inverted. That happens for example if the geometry is inverted(scaled by negative values) at some point during modeling to mirror it. You should go to edit mode (tab), select everything (a) and recalculate normals before baking them. The hotkey is ctrl+n in 2.79 and shift+n in 2.80. You can access functions related to normals in the ctrl+f or Mesh > Faces and Mesh>Normals menus in 2.79, while in 2.80 there is a dedicated Normals menu alt+n. 

However in this particual case the issue is the normal map made with overlaping UVs for symetrical geometry. If you have a look at a normal map of a symetrical object, the normals are not mirrored - they differ on each side so if you mirror them you get an incorrect result.