I have created a simple piece of code for adding vertices to a mesh by clicking on it. The vertex is added to the point of the mesh which is clicked. This works very well the first time. But the second time it fails to get the correct triangle from RaycastHit.triangleIndex. Here is the code, including a hack which actually makes it work. (The fact that the hack works also eliminates the possibility of this being a corrupted mesh.)

public void AddVertexScreenPos(Vector2 screenPos)
{
RaycastHit hit;
if (HitScreenPos(Input.mousePosition, out hit))
{
if (hit.collider.GetType() == typeof(MeshCollider))
{
MeshCollider meshCollider = (MeshCollider)hit.collider;
Mesh mesh = meshCollider.GetComponent<MeshFilter>().mesh;
List<Vector3> vertices = new List<Vector3>(mesh.vertices);
List<int> triangles = new List<int>(mesh.triangles);

vertices.Add(meshCollider.transform.InverseTransfo rmPoint(hit.point));

int[] triangleHit = GetTriangleIndices(ref hit);

triangles.Clear();

triangles.AddRange(RemoveTriangle(hit.triangleInde x, mesh.triangles));

for (int i = 0; i < triangleHit.Length; i++)
{
triangles.Add(triangleHit[i % triangleHit.Length]);
triangles.Add(triangleHit[(i + 1) % triangleHit.Length]);
triangles.Add(vertices.Count - 1);

}

mesh.vertices = vertices.ToArray();
mesh.triangles = triangles.ToArray();
mesh.RecalculateNormals();
mesh.RecalculateBounds();

//The following line should update the meshCollider, but it fails.
//meshCollider.sharedMesh = mesh;

//Hack. This fixes the problem
GameObject go = hit.transform.gameObject;
GameObject.DestroyImmediate(meshCollider);

go.AddComponent<MeshCollider>();
//Hack end.

}
}

}

Any suggestions?