I wouldn't say any of these have been terminal, but I might reconsider some of the following things I did, if given the option:
Chose a major based on what I felt like spending four (...okay, five) years doing, rather than on what I thought I might enjoy spending any significant portion of my working life doing. In fairness to my past self, I don't think I had enough information when I was 18 to have made anything like an accurate guess, but maybe it was also a mistake to fail to attempt a solution to that preliminary problem.
Moved in with a significant other even though I didn't want to. (For the record, it turned out great! But there were some hair-raising moments for a while.)
Stayed in a job that sucked and didn't have any obvious prospects for improvement. Didn't try very hard to find a new job while I was there.
The overarching theme here: not knowing, not trying to figure out, not asking for, and not making/executing plans to achieve what I really wanted.
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.)