This particular adage is neither new or proprietary to Yung.  I believe that some form of this advice is published in every project management reference ever written.  A common form is "Avoid goldplating."  Yet, in spite of the wisdom of the thought, we, as project managers continue to fall into the trap.

Sometimes, crossing the line is accidental.  We just mean to polish one last thing.  Sometimes, we neglect to train those who work for us how to deflect that client request.  We teach our team members to never say "no" to a client request, then we wonder why the scope has gotten out of control.  Clients and customers are very smart.  They zero in on those team members that will always say "yes" to them.  And, our team members sincerely believe that they can give that "little extra" without impacting the project.

It is prudent that we we tell our team members to never say "no" that we teach them the appropriate way to deflect that scope-increasing request. 

• "Yes, but who is going to pay for it?"
• "Yes, that is possible, but I have to check with my team leader.
• "Yes, sure.  Can I have your Change Control reference number?

There are probably an infinite number of variations on the "Yes, but ... " answer.  The trick is becoming comfortable with the implied challenge.  The project manager must remain vigilant throughout the process.  If this vigilance about scope is relaxed for an instant, a project may easily go out of control.

The beginning of Verse 20* of the Tao hints at this verbal judo.

"The difference between a formal "yes""
     and a casual "yeah" -- how slight!
The difference between knowing the truth
     and not knowing it -- how great!

*Tao Te Ching, The Definitive Edition, Lao Tzu with Translation and Commentary by Jonathan Star, Jeremy P Tarcher/Penguin, New York, 2001, page 33