The biggest mistake I think I made in my life was not a conscious decision, but rather an opportunity I would have known I was missing if I had been paying attention. I should have applied for early admission to college, instead of going to my senior year in high school. By the end of my junior year in high school, all the friends I wanted to spend time with were college students anyway. Spending another year with my high school classmates was not enjoyable, but the real problem was that I didn't learn anything useful in the classes I took in my senior year, and I had so much free time and so little structured expectation placed on me that that year really solidly ingrained habits of laziness that had been building up to that point. If I had gone early admissions, I not only would have not wasted a year of my life, I would at least have a chance of still having some motivation.
The next decision I'll write about would have been in the Spring of 1990. I had put down an offer on a house in southwest Champaign, and between the time the offer was accepted and the closing, I lost my job. I could have carried through with buying the house -- all I would have had to do would be to perjure myself in signing the form at closing that there had been no significant change in my financial status since I had applied for the mortgage. I would have been a homeowner a year sooner. If I had held that place a few years, it would presumably have appreciated a fair bit in value, and I could have used the money to move up to a nicer place than I now have, or perhaps I would have just settled down in the city life. I don't know that I would have been better off, but it would certainly have been different.
Probably the most significant road not taken in my life was in 1993, when BAO, the company I was working for at the time, was bought out by Microsoft. Part of the buyout agreement was that Microsoft had to offer all of the developers positions in Redmond. Every part of the offer they made me was enormously attractive financially -- except for the base salary. I did not think it would be possible to qualify for the mortgage on a place I could live with 3 large dogs, and I was not at all confident that I would have been able to fit in with the operation and be retained. But I've never been able to look back without wondering what would have happened -- the 1200 options included with the employment offer alone could have been $1 million in my pocket (before taxes, at least) if I'd sold the stock at the peak. And the house I would have bought, assuming I could actually buy one, would have at least tripled in value in the subsequent decade. (Houses in the country just east of Seattle were almost reasonably priced in 1993, but they must have gone through the roof by now.)