Self-made millionaire David Bach made the biggest money mistake of his life on the first day of his freshman year at USC, when credit card companies were tabling in front of his dorm.
Representatives were giving away bike locks and sweatshirts to students who signed on and, "before I knew it, I had three credit cards," Bach tells CNBC Make It.
"I did everything wrong," he continues. "I used those cards — I told myself I wouldn't, only in case of emergency purposes — but I used them. I maxed them out. I started to make only minimum payments."
By sophomore year, he was $5,000 in debt. "It was the worst financial mistake of my life. And it was painful."