In this model, I have used extrude, loop cut, inset for creating the model and the modifiers that I used was boolean, mirror modifier along the y axis and the subsurf modifier. The problem is why there seems to be a line between the mirror and the object where I lined it blue? Is there anyway to solve this ? At first, I think maybe there is a face between the model and the mirror but I have deleted it so I think the problem lies somewhere else.
I have made the vertices along the center and scale their 'y' location to 0. And I have found some problem that some parts of the meshes would go beyond the center even if I have already clipping turned on. I cannot find any face that is between the mirror and the model so I don't think that is the problem and I also have removed the doubles