Strange line artifact in a mesh

Question

After merge the leg points with the body points I get this strange line in the intersection. Why?

This is the mesh:

This is the effect with shade flat:

And this is the effect with shade smooth:

This is the file for review it.

Thank you!

Answer 1

There is a fully connected face "inside" your leg - meshes like this are known as Non-Manifold (they have stuff in them that shouldn't be there) - Blender works best with manifold geometry ("hollow" meshes) - delete the offending (interior) face and you're good:

Answer 2

You have an internal face - that is, a face that runs inside your mesh from the verts seen on the outside.

You can see this if you go into X-Ray mode:

Delete it and you'll be fine.