<prompt> Human: It could be a car, or a gaming rig, or an expensive habit. Maybe a college major, career choice, a job you took. Or a stock purchase or a house bought in 2007. Maybe bailing out a family member or enabling them, or picking the wrong spouse, or getting pregnant at an inopportune time... Whatever it is, you now have regrets and consider it your biggest financial mistake and would do it differently if you could.Edit: Wow, thank you everyone, for your responses! To be open and honest and willing to share your mistakes with the rest of us so we all can learn from one another, that is a truly priceless gift of sharing. A lot of people seem to have the impression or at least they grumble about it that rpersonalfinance is circle-jerk where everyone is 20-something making six figures. But in reality, many of us have made expensive mistakes and have paid dearly for them - we just dont talk about it much - and thats why were here.  <response>: Not sure about biggest, Im an old guy and have been in the game a long time. The most recent one that still stings me is an ASX stock I had in about 2004 for a realestate website. ASX:REAI was planning on a long term hold and adding to a very small holding in them over the years, but during that year we had some major home renovations done.According to my records I had 24688 shares at that time, which I sold for an average price of 90c. Payed an average of 27c. Approx 6.5K into 22K and was extremely happy with the return for approx 15 months hold time.So anyway 10 years on, that stock closed for 48.45 today.48.45 x 24688 units 1,196,133.60When youre in this game long enough, it will happen more often than you think. Doesnt make it any less sickening. 