I do see what appears to be a twisted face which is what non-planar is. I also count 12 verts. But I only see 5 not 8 faces. So either three other faces are selected someplace which would not make sense actually becaue then you would have more then 12 verts

So I am guessing you also have laminar faces which are faces on top of each other and non-manifold geometry which is harder to describe but non-manifold would be things like faces that cannot be produced in real life such as two faces connected at one edge but flipping a face so it has normals on opposite sides or two faces connected at a single point.

To be honest that geo is so simple you might just find it fast to delete and create them.