I'm looking for feedback on what financial lessons we generally don't learn at home before age 18.

Mine came when a benevolent person at the first company I worked for TOLD me I had to sign up for the 401(k). I resented giving up something from my paycheck so signed up for the lowest percentage. When the company matched I realized I needed a higher percentage to get all the match - changed the percentage and learned how easy it is to figure a way to live with dollars I never saw.

I was embarrassed to have grossly overestimated my transit spending (living and working Manhattan) and learned the hard the hard way about deadlines and pre-tax transit accounts. I managed to keep my loss below $100, but my first time dealing with that was not the most responsible. But it was a good lesson learned...I've been good at managing it since then!