There are many things a physician should never say to a patient.
"This won't hurt at all," is one such statement.
Many physicians seem to overestimate their ability at performing certain procedures painlessly. The fact is, many procedures do hurt, at least a little. I can easily attest to that. I even find having my blood drawn to be objectionable at times.
The recommended terms to use are "this could be a little uncomfortable" or "this will hurt some, but it'll be over before you know it."
I still laugh about a former orthopedic colleague. He would always inform patients that the office procedure he was about to perform "won't hurt me a bit, so don't worry."
It served as a good tension relief, for the few patients who got it, when they could laugh together.