Going crazy with credit cards and getting into debt in my early 20s. Especially when I wasn't making much to begin with.
Remember having to borrow money from my pops, and a friend right before me and him went to Vegas on our second trip there.
Wasting too much money when I was around 18 and 19 by going to play poker with that same friend when it was getting popular. Those $50 every other day or so could've been used more wisely. Some days were wins, some were losses, all were unnecessary.
Over 50% of the time my friend just wanted to go gamble. And while he got some success with poker, I didn't have much. Barely see him these days but he always talking about meeting up at the casino, I try to save my money instead.
Staying at a job that I wasn't growing in for too long and not earning more than I should have. Leaving was the best decision I ever made.