My first exposure to combatives was over 20 years ago, around age 12. My maternal grandfather was visiting and I had just come back from a Hapkido class (or TKD?) and he asked to see some of my moves. I think I did a kata for him and such. My grandfather was in the Canadian Army during WWII and mentioned that they did a little hand to hand training back then. He asked if I wanted to see some, I foolishly said sure. I believe I dropped back into a backstance, while assuming my best don't fuck with me face. As soon as my foot was planted he scraped his heel down my shin (OUCH!), my hands dropped and he chopped me in the neck. That was the end of that sparring session.
My grandfather had never done any martial arts training before or after his army stint. My grandfather (in his 60s) was able to drop me in seconds with one of a handful of techniques he learned around 45 years ago - in an afternoon training session. At the time I was pretty impressed by what he did, the "cool technique." Looking back on it now, I am absolutely astounded not by the technique but at his retention of it. For him to effectively apply a fighting system that was briefly learned - 45 years ago! - and rarely (if ever) practiced, is truly amazing.
My Grandpa was a great man, we all miss him very much.