Leaning engines is a mystry to me and I avoid it where possible - being mechanically challenged I'm convinced that the engine will stop
I've had 2 pilots in 4 days pull the mixture when they thought they were pulling the carb heat - one on the ground during powerchecks, one running into a practice stall. I've only ever had it done to me once before, about 8 years ago, now twice in the same week.
On the general subject of PPL knowledge - I meet many new PPLs and they don't know the exactly same things I didn't know (but needed to) when I was at their stage! I think many instructors say they wished they'd known (whatever) when they were merely PPLs, and then the instructor-instructors will say they wish they had known something else when they were only instructors, and so it goes on.