No, this post is not about the use of illegible fonts, invisible or clashing color schemes, text-filled slides, excessive animations, or even attempts at humor involving anatomical references that some of the men in the audience think are funny but most of the women do not (proving that women have no sense of humor). In the past 8 months, I have been in the audience of two different talks that attempted the latter, and many more that utilized the other design options.
But let's ignore those issues for now. This post concerns one specific aspect of how we answer questions after a talk: defensive question-answering.
Most of us have seen examples of defensive question-answering (or non-answering), but I saw a new variant on it in a recent talk. More than once, when the speaker was asked if he had done X or Y or thought about Z, he replied:
If I'd done/thought of that, I would have mentioned it.
That is a bad answer because it is a non-answer that will impress no one. Some people will assume you don't know the answer, and therefore possibly that your approach is narrow and/or you are not a careful researcher or deep thinker.
Perhaps those inferences are too harsh, based on what might have just been a throwaway line, but I couldn't think of a good reason for the repeated refusal of the speaker to answer thoughtful and polite questions by people seeking to better understand his research methods and results.
Why not answer the questions politely and sincerely, even if you think they are stupid? You could say something like "No, we haven't done that because..". You could even say "No, that won't work because.. [explain]". Be professional, be mature, and toss the question back with some information attached to it if at all possible.
I think these defensive answers would be unimpressive no matter how great the experience or fame of the speaker, but I thought it particularly unwise for an assistant professor to give these non-answers instead of attempting a thoughtful or at least more complete answer. If you are trying to build your reputation, build it from substance, not swaggering remarks.
Even if a question is an aggressive attempt to undermine you or show that your research is incomplete, there are numerous thoughtful and suave ways to answer questions of this sort. Last year, I saw a young professor parry the very aggressive and obnoxious comments of a Big Professor after the young professor's talk at an international meeting. It was impressive. The YP was very calm and polite as he responded to the Big Professor's hostile comments, showing that he (the YP) was right and the BP was wrong. The audience gave the YP a huge round of applause after the questions were over, and many people considered the exchange the highlight of the meeting, in part because of the cool and professional way that the YP dealt with the situation.
If you really have absolutely no idea how to answer a question, you could at least say "No, I haven't done/thought of that. I/we first tried [mention what you've done] because.. etc.". Perhaps you will be repeating what you already said in your talk, but that's OK -- you can turn the question back to what you have done and why you did it, perhaps clarifying the motivation, methods, and results of your research.
You don't have to give a brilliant answer to every question, but a serious answer with substance is preferable to an aggressive non-answer.