I had high school and college experience speaking in front of large audiences, but the very first "professional" talk I gave was on Huygens' empirical argument against Newton's claims about universal gravity in the Summer of 1995 just before I went to graduate school at an international Huygens conference in Leiden, the Netherlands. My parents (and some of their friends) were in the audience. It was a co-written paper with George Smith (one of my ongoing academic mentors), and, while I had done not trivial archival work in researching it, it was at that stage of drafting really his paper. (George was a keynote at the conference, so he had decided I should present this material.) We had made photo-slides (you know the ones you put in a projector). I practiced the talk (i.e., reading George's paper) on somewhat crappy overhead sheets. (You know, the ones that Brit dropped in front of her 200 classmates.) The night before I couldn't sleep, and even checked out of my hotel to a different one with more comfortable beds, I thought. The next day, during the break before my talk I discovered to my horror that the slide projector in my room had burned its lens (or something like that). No other projectors were available, so on the spot we decided that I would use the practice transparencies. As I started speaking (rather rapidly and nervously) to a crowded, standing room audience (George had been talking up the topic--we were, in fact, presenting about 25 different discoveries), I was loudly interpreted by three elderly guys in the front row (think of Stattler and Waldorf from the Muppets)--they were protesting to the chair of the session that my talk should be in Dutch (because, after all, we were in Leiden). After some back and forth they were overruled by the Chair (it was an INTERNATIONAL conference after all). I later learned they were antique dealers interested in Huygens clocks. ONE OF THEM FELL ASLEEP DURING MY TALK in the front row.
After five overheads or so with text, I put up a map (of African Atlantic with voyages drawn on it); murmurs from the back of the audience--the drawings were invisible. I am not sure what I said, but I decided to plow ahead and talk faster. I was later told that I was barely intelligible because I was turning all 'th's into 'ff's--something I do anyway. (This talk was written by George so there were a lot of 'th's [I avoid them now].)
Out of the blue I got interrupted by a very fierce looking man who insisted that I had made an error and kept yelling that I didn't know the amount that gravity at the equator is diminished. (The correct number is 1/289.) I knew that, of course, but I was reporting Huygens' much larger number (from rotation alone). At this point, George who was sitting toward the side of me lost his cool and started yelling back at the guy. (In my memory, but I am not sure it can be trusted, he also pointed his laser-light that he always brings with him at the guy.)
At this point I take of my sports-coat (yellow Armani, although it is an ugly green now), and a complete and utter Stoic calm came over me. (The only other time I felt like that was when Dan Garber's beautiful Rotweiler, Karma, bit [or so I thought] off a part of my nose [in her defense, I had awoken her] and all I could see was blood, while Allan Gabbey kept yelling, "we should shoot the dog now!" while Garber kept telling him to shut up.) I read the remainder of the talk straight through. In Q&A I answered a few questions, and that was that.
After the talk, the great Huygens scholar, Joella Yoder, told me that in print nobody would remember these silly events and all that would remain were the brilliant discoveries. (Little did she know that the talk is still waiting to be published! It's been accepted ages ago.) One of my mom's friends who was present, a medical scientist, assured her I would go on to a fine academic career.
There are, however, two amusing post scripts to this event. PS I did publish a small piece on one of the (very rare) maps that I found in an archive (another great story for some other time) discussed in my talk. A few years later I walked into an antique-maps store in Amsterdam, and asked them about the value of that particular map. I was told that for a long time it would have been a couple of thousand guilders, but now after some academic had written on the map it was probably twenty times more valuable!
PPS a few years ago, I was giving a talk on Newton at UC Santa Cruz. Halfway through the talk I got interrupted, and I recognized the voice immediately. It was that guy from way back when, the great , Michael Nauenberg, a physicist and very accomplished historian of science. This time, I kept my cool, explained why his assumptions were based on dated research, and moved on with my talk. Petty as it is, it was sweet vindication.