If you are going to interrupt a speaker to criticize their work, always shoot for the head. And only do this after you have seen it done successfully several ti

If you are going to interrupt a speaker to criticize their work, always shoot for the head. And only do this after you have seen it done successfully several times. That is what I was taught in grad school. It's rarely observed. More specifically: If the talk is informal and the speaker invites discussion, then by all means discuss. But only discuss things that are of general interest. Things that are of interest to a small number of people should be discussed offline. If the talk is formal, then the only times to interrupt are if the slide is unclear or there is something that needs clarification, and is of general interest. The general principle is to discuss things offline except in the face of strong evidence to the contrary. Unfortunately, there are a few loud types who don't observe this principle, to everyone's detriment.