1) Buying and eventually selling a house i didn't like, twice. Doing this always costs $20k each time.
2) I once lost a few thousand dollars buying put options on Netflix around 2011 or 2012. My rationale was that their content sucked (it did at the time) and the networks, HBO, etc. who were withholding content from Netflix would surely launch streaming services within a couple of months. Instead, years went by with no significant competition and Netflix invested in creating some of the best content anywhere. I went short the best performing FANG stock of the last 20 years. :)
3) I also bought puts on gold (GLD) in about 2011 or 2012. My reasoning was that the price was rising even as there was no inflation in sight. It was an expensive lesson on why gold has nothing, zero, zilch to do with inflation.
4) Investing too conservatively all my life, and letting the doom-news dictate my macro view. Even when I was several years from any possible retirement, I kept large amounts of cash and bonds and various small speculations, but was afraid to go all in on the S&P or Nasdaq as a young person should. I'd be a millionaire now had I only been willing to take the chance on a fraction of the money I must take the chance on now. I need to find a way to chill and "let it ride".
5) Generally taking the easy and low risk path through life, avoiding hard work or the possibility of anything going wrong, etc. Sticking with jobs that were going nowhere instead of looking for / seizing riskier opportunities. Trying to find work I wanted to do, which paid poorly, instead of work that could have retired me in 10 years. Now I wish I had put the pedal to the metal and taken some chances. I know several people who stepped out of their comfort zone and suddenly started making six figures in their 30's, but that seemed too hard for me. Well, guess what, working another 20 years is hard too.
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Yet overall I've made mostly good decisions where it counts. I never started smoking, never got a DUI or even a serious wreck, never tried several addictive substances that were offered to me at various points, never got in trouble with the law, backed down from a couple of fistfights, never made payments on a car in my whole life, managed to avoid divorce so far, avoided losing money on several cons, and took good care of my teeth. It's impossible to play a perfect game, so maybe I'm lucky most of my dumbest moves have been financial - I'm a good saver so I recover well from these goofs.