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:

g square.mesh

v 5 5 0
v -5 5 0
v -5 -5 0
v 5 -5 0

vt 1 0
vt 0 0
vt 0 1
vt 1 1

vn 0 0 1
vn 0 0 1
vn 0 0 1
vn 0 0 1

f 1/1/1 2/2/2 3/3/3
f 3/3/3 4/4/4 1/1/1

What it looks like after import into Unity:

g square.mesh

v 5 5 0
v -5 5 0
v -5 -5 0
v 5 -5 0
v 5 5 0

vt 1 0
vt 0 0
vt 0 1
vt 1 1
vt 1 0

vn 0 1 0
vn 0 1 0
vn 0 1 0
vn 0 0 1
vn 0 0 1