Great thread, Rob!
My first lesson after certification (because there were several during) was having a freeflow in about 30 feet of water. Although I had been taught, of course, that you could breathe off a free-flowing regulator, I was entirely unprepared for the decreased vision and the horrible NOISE you are surrounded by when it happens. Although I accepted my buddy's regulator to breathe from, and we got to the surface without completely losing control, my buoyancy stank. That was my first tip that I'm dangerously dependent on a visual reference, and it was also the first object lesson in how anxiety changes your breathing and complicates buoyancy control. I also learned how darned fast a freeflow empties even a big tank.
The next one I remember was sailing off the top of a wall with my friend Bob, and putting my arms out wide to fly, grinning widely, and . . . flooding my mask. For some reason, I could NOT get it to clear, and after several attempts, I got vertigo. It was very dark, and I thought I was somersaulting, and I realized I had forgotten to ask an important question about the site, which is how deep the bottom of the wall was. In the dark, unable to see, unable to communicate, and subjectively sure I was tumbling wildly through the water, I felt, for the first time since OW, the urge to bolt. And it was having had that experience before that let me pound it down and stay in control until I could salvage the situation. Lesson learned was that incipient panic can be felt before it's out of control, and can be averted.
The third one I am going to talk about is not a basic scuba dive, but the lessons are pertinent. I had flown back to Florida to dive with some friends at a place called the Mill Pond. We were going cave diving in a cave I had dived before, Jackson Blue. However, the prior time, it was in drought conditions in Florida, and the flow out of the cave was very mild. This time, it was like trying to swim down a firehose. I was in the lead, and I negotiated the first part reasonably well, but I was working very hard and breathing hard. We did a big drop through a slot, where the flow was stronger yet, and got to the bottom. I was looking ahead to figure out how I was going to get from where I was to the next place I could hold on for a bit, and I realized I didn't want to be there. I wasn't scared, but I wasn't having fun, and I didn't want to keep going. Now, behind me was my husband, who had also flown 3000 miles to do these dives, and behind him our friends, who I was also going to deprive of a dive if I called it. But I seriously just didn't want to be where I was, doing what I was doing, and I remembered the principle that anyone can call a dive at any time, for any reason, without recriminations. And I thumbed it. The astonished look on the faces of my companions was hard to see, but we turned around and went out. I explained what happened at the surface, and we all rested, regrouped and changed leaders, and went back in in a little while and had a lovely dive.
Lesson? There is ALWAYS another dive, so long as you come back from this one. If you don't want to be where you are, go somewhere you want to be. And don't let worry about what other people want or will think stop you from backing out of a dive where you aren't comfortable.
Ten years . . . I could write tons more, and I have posted some of my blunders here over the years. I guess the final "lesson learned" is that we all make mistakes, and the important thing is not to assign blame for something, but to learn from what didn't work well, and change your response in the future.