2. Not having an automated savings system. Don't save what's left over. That rarely works. Pay yourself first, with an automated system.
3. Not having health insurance. While it looks like an unnecessary expense, you must have some type of catastrophic coverage. Consider an HSA as a minimum.
4. Buying things to impress other people. Don't do this. Develop an attitude that doesn't wrap your self-worth in material things.
5. Not having an investing education. This should be a continuous, life-long learning process.