I have a simple .obj file with a square made of 4 vertices and 2 triangles.
Importing the file into Unity with the FBX normal import settings set to "Import" results in a mesh with 5 vertices and 2 triangles! Also, 3 of the (now) 5 vertex normals are 90 degrees off from the input normals.
Input to Unity:
What it looks like after import into Unity: