When I was in my early 20s there were a few personal finance things like this I didn't understand. Here are a couple examples:

1. I refused a credit card limit increase because I thought that it was a trap to get me to spend more. I didn't understand FICO scores are partly determined by "amount of credit" and "credit utilized". I likely cause myself to later spend more money on higher interest rates when I got a loan because my credit score could have been better.
2. I was mildly upset I got $1000 as a bonus instead of a pay income because I thought that bonuses were "taxed at a higher rate" when instead they are simply "withheld at a higher rate" and it all washes out in the end when you pay your taxes at the end of the year.

But I grew up and learned these things in my 20s and didn't carry those mistakes into later adulthood.