Not living on campus while you attend college, and also not having a car.
My girlfriend was required to live on campus for her freshman year- when she moved out, her rent was $300 cheaper, the apartmant was twice the size, and it also seemed nice to have a freaking sink, and not leave the dog at home. And I would expect the cost of a monthly bus pass to be exceeded by gas alone. So why was this a mistake for you or your friend or whomever? It's not at all obvious to me.
The friend in question had to spend more than an hour a day carpooling to get to and from campus. (I don't think there were any buses from where he lived). This meant tons of waiting around for rides because college schedules are sparsely scattered throughout the day. This also meant going to office hours was a pain, and he couldn't (like I, and most other engineering students I knew could) get into labs in the middle of the night to finish assignments that frequently took longer than expected.
That said, this probably isn't nearly as big a mistake in most people's cases, and I'm updating my original post to reflect that.
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.)