Have you ever tried Create Spline from Selection, and got a spline with stubborn vertices which just don't go smooth no matter in Smooth/Bezier/Bezier corner? I had been troubled by this since long ago but never really find a solution till today....
yeah, it's pretty simple to solve it:
1) select the spline, goto Segment
2) right-click, choose the "Curve" from the quad-menu.
Voila, the spline is now smooth! ^^