Others may simply be incorrect in their confident assertions.
This example is closer to the former than the latter:
CBC News - The egregious actions and abysmal failures of Dr. Charles Smith have disgraced the entire medical profession, Ontario's doctors' college said Friday in a severe reprimand to the discredited pathologist. Smith, who was not at the College of Physicians and Surgeons of Ontario to be chastized in person, was stripped of his medical licence last month.
Years after it became clear his mistakes led to several people being wrongly convicted of killing children, the college came down hard on Smith in its chance to express its "abhorrence" of the doctor's conduct.
"Your transgressions were egregious in nature, repulsive in result, and caused irreparable harm to many innocent victims," panel chairman Marc Gabel said to the empty chair reserved for Smith. "You had a duty to the public, to the administration of justice and to your profession. Your failure in all of these respects is abominable to this panel, to your fellow physicians and, as importantly, to the public." ... [LINK]
This warning about "expert witnesses" absolutely applies to the subject of tasers.