Suppose you are a discussant for a conference. You see a flaw with the paper to be discussed.
When, if ever, are you supposed to tell the presenter in advance?
I feel stupid playing "gotcha" and potentially embarrassing the author in front of an audience. On the hand, I don't want the author to build defenses against all my points into their presentation and make my discussion seem irrelevant to the audience (who probably didn't read the paper.)