I'm fairly new to Blender modeling and I'm following a Blender Guru Chair Modeling Tutorial. At Some Point, I realized there were weird shading artifacts in my geometry, as you can see in the picture.
I'm pretty sure there are no unnecessary vertices overlapped and the face that has shading problem has a flat face. Also, there's no Ngon or Tris in my mesh.
Couldn't really wrap my head around where this shading artifacts comes from.
The problem is that your subdivision surface modifier is creating some strange geometry in the right leg of your object. The shading artifacts are due to overlapping vertices and extra geometry inside the leg. I would go to the wireframe view and look very carefully inside that leg, and try to delete the overlapping/redundant geometry. This is an illustration of what I mean:
Tellingly, when I remove the subdivision modifier, the weird geometry in the right leg goes away. So it's creating extra geometry due to bad vertices.
The problem seems to come down to these two extra vertices:
Delete them and see if the artifact goes away. You might have to experiment to see how to clean up the geometry in that area (by deleting extra or unnecessary vertices) and that should get rid of your artifact. Topology is always a tricky topic, but creating a clean mesh is absolutely vital. Another possibility is forget the Subdivision Surface modifier and try the Bevel modifier instead. that will smooth out the sharp edges in a more predictable way with odd geometry.
Maybe look into CGCookie's basics course (which is free) as they cover problems such as these and how to solve them.
To add to ArchTeryx' answer, you have these 2 faces inside your object, remove them:
