Worst Financial Mistake

My worst financial mistake was not exercising my stock options when they were up about $5 a piece. I lost about $10,000 due to my greed. I kept thinking the options would go higher. By the time the trading window open for employees to trade the stock was below my exercise price. a year later they expired. I did not make that mistake twice, this time I exercised my options when I got the chance. My profit was only about $3700 after tax but the money was secure in my account. Greed is very hard to overcome when it comes to stock and money in general. My lesson learned was that money in the bank is worth a lot more that potential profits. What is your most expensive Financial Lesson?