It reminded me of something that happened on one of the first commercial courses I ran as I was beginning to make the move out of full-time academia into training consultancy.
Then, as now, my policy was and is not offer any firm advice that can't be supported by at least something in the way of research or observation. In reply to a question from one of the delegates, I must have said something along the lines of "we don't know".
The reason I remember it so vividly is because there was considerable discussion over lunch about the fact that I was apparently the first consultant they'd ever come across who'd actually admitted that there was something he didn't know.
This, coupled with something I'd seen a few weeks earlier, gave me the confidence to carry on admitting that there were plenty of things I didn't know.
It was the case of another training consultant running a presentation skills course in which he had criticised one of the participants for failing to switch the overhead projector off and then on again every time he put on another slide. But this particular particular pupil wasn't convinced:
'(he) complained that, when he was in an audience, he found it extremely annoying when speakers kept turning the projector on and off. He even dared to ask why it was deemed to be good practice. After some initial hesitation, the consultant quickly recovered his capacity to sound authoritative in the face of mindless ignorance with the memorable line: "because it's correct"' (Lend Me Your Ears, pp. 9-10).