Not taking the time to apply for good internships during college.
I fell into a similar failure mode. My program requires internships, but I hate the job-search process, so when I got an offer from a good company, I accepted it pretty quickly, even though it was so early that many of the companies that I had applied to hadn't started their interviews yet. I forgot to ask myself which aspects of the job I wouldn't like, and was too impatient to stop searching.
(Disclaimer: if anyone happens to know where I work, it's really not a bad place. It's just not a good fit for me.)
This is a repository for major, life-altering mistakes that you or others have made. Detailed accounts of specific mistakes are welcome, and so are mentions of general classes of mistakes that people often make. If similar repositories already exist (inside or outside of LW), links are greatly appreciated.
The purpose of this repository is to collect information about serious misjudgements and mistakes in order to help people avoid similar mistakes. (I am posting this repository because I'm trying to conduct a premortem on my life and figure out what catastrophic risks may screw me over in the near or far future.)