Hi Nikola, you have to zoom in quite closely to the tip to see it and also it helps to turn on the surface control points but here you can see how the surface folds back over top of itself in a small area near the top there:





When a surface folds back over top of itself like that in a small area, it means the surface normal chaotically bounces from one side to the opposite side, that's what's making for the odd shading there.

It may be possible to fix this up ok by selecting those control points at the top and collapsing them down to a single point instead their current semi-chaotic arrangement, that's what I've done with the attached updated model.

If you've constructed that surface using something like Network or Sweep you might zoom into that area in your original construction curves and see if the curves are not quite touching each other.

- Michael