Not maxing out retirement savings early in my career. Compound interest FTW.
If it makes you feel any better, I read a clickbait article this week on why young people shouldn't save for their retirement. The gist is that they can easily make up the difference in their high-earning older years, so it's not worth the sacrifice of trying to save in their low-earning years, especially when the market is flat. I don't necessarily agree, but it is a perspective I hadn't heard before.
1) that your spending needs won't increase as your salary does.
2) that your salary meaningfully increases over time (more true for the top 15% than for 85% of others)