Broken: Blender v2.80 (sub 75)
I have found something weird while trying to export VertexGroups.
If you look at the image above and imagine a dice, you can see two groups representing the dice's faces (group 'Six' facing up and group 'WTFOne' facing down ; with face 'Six' at zero to avoid ambiguity).
According to wikipedia's OBJ specification the group 'WTFOne' should olny contain a single face, not all the faces left except the face inside group 'Six'.
As you can see, things gets weirder if I delete 'WTFOne' group
Also, what does SmoothShading ('s') do here? I mean, just inside the first VertexGroup, where 'f 1 5 7 3' should normally be?
Maybe I miss something about null values?