1) Kept too much money in company stock acquired through stock options. I should have exercised options quicker, sold the stock immediately, and been more diversified.
2) Built a large new home near the peak of my career. While paid for, we don't need the space and upkeep in retirement. Downsizing and selling takes time away from enjoying retirement travel. Keeping our smaller, paid for, home would have prevented us from acquiring more "stuff" to fill the bigger house which we don't need today.
Retired at 57. Now a happy camper!