I am looking at shelves of books and diving manuals that I've read over the past 41 years in the sport. Two even have my name on them! The older books are my favorites. They seem to contain the most in-depth information and are also the ones I remember most due to the principle of primacy. Books also remind me of the classes I have taken, the instructors who taught them, and fellow students whose ingenious solutions and complete foul-ups served as learning experiences. As I age, I really am starting to forget a lot. Like my mask is already on my face when I'm looking for it. There's that saying, "He's forgotten more about diving than most of us will ever know." In some ways that is true because things that were once taught in diving courses are no longer taught. In other ways, it is not true because so many divers are learning to use technology that I have only had rudimentary training in (such as rebreathers) or no training (such as sidemount).
How about you? Do you have any favorite academic or practical aspects of training you learned but never had to use? Or, perhaps something you never thought you'd have to do, but you did?
I recall an instructor who had me learn to breathe directly from a tank valve, in the context of a solo diving situation in which either I only had one tank with one first stage or all first stages available had failed. I was instructed that it was likely this is a skill I would never actually need in real life, but we're going to learn to do it anyway. Wait, that instructor was you, and you were right; I have never needed to do this in the 15 years since. But it does give me a strange feeling of added security to know that it can be done, I can do it, and I have done it (and it sucks). It was also an excellent reinforcing impetus to only solo dive with fully redundant and adequate gas supplies (i.e. 2 of everything minimum, and 3 is better) to truly ensure I'll never need this skill.
Some other skills I've not needed but am very glad to have learned in case I ever do need them are (and I will skip all the ones others have already mentioned):
Lost line, no lights lost line, and most other related cave scenarios
Calling DAN for assistance, advice, medical opinion, etc.
Honestly, it's getting hard to think of more, because there are many more things I have used because I like to practice them or that I've only needed once than there are things I've NEVER needed or done.
My next reply will be about some skills I surprisingly have needed or used that one would think should be on the "never did" list. But first, one last "never needed" skill I learned from you. It's a complex scenario I like to call "Buddy lost all 3 lights, and my three lights, and his mask, and his backup mask, and my backup mask, and my mask, and his deco bottle with some deco to do, so we had to buddy breathe my deco bottle using touch contact communication while maintaining contact with the line to the exit" Now that was some big fun, but I'm pretty sure that's never going to actually happen. Thanks for the memories.