I can't think of much that I have done that I can be certain I won't do again, though some are unlikely.
1. Changing the engine in a car myself.
2. Moving house.
3. Getting married - had the same wife for over 40 years, we are unlikely to divorce, and I don't think I'd want another wife if she dies before me.
1) DIY clutch replacement in a rear-wheel drive car.
Or put another way, never again will I find out exactly how heavy certain car parts are by putting the vehicle on axle stands out in the street, crawling underneath and unbolting the propshaft and
gearbox just above my chest/neck/head.
2) And if I did ever get tempted again, this time I wouldn't leave the
release bearing in place just to save £50, only for it fail less than 3 weeks later. Damn.
I used to be quite ambitious with my car maintenance. 30+ years later, bulbs and wiper blades are my limit!