Not autobiographical, all but one are people I know or have known personally. Which is not to say I have avoided all of these mistakes, but none of this is telling my own story. The ones about reasons for choosing college majors are speculation about other people's motivations, but are obviously mistakes whether they are the true reasons for people's bad choices or not.

Waiting through several years of depression and suicide attempts before telling anyone or getting it treated.

Not living on campus while you attend college, and also not having a car. (Edit: it has been pointed out that this often isn't that big a mistake. These are the circumstances under which I would call this a big mistake).

Not getting good enough grades in high school to get scholarships in college.

Going several thousand dollars into debt to pay for your first year of college classes, and then dropping out.

Living under parents who expect you to do several hours of manual labor per day while attending college, depending on them for rides to and from college, while sinking most of your income and lots of your time into construction of a cabin, far from said college in your hometown, which said parents own the deed to, while you do not have a car.

Registering as an engineering/nat science major in college when you are not good at math or cannot deal with hard work.

Having babies before you have finished all the formal education you want to get.

Not taking the time to apply for good internships during college.

Not taking enough time to apply for colleges during high school.

Not paying attention to deadlines for fellowship and grad school applications in the fall a year before school starts.

Not taking the GREs soon enough before you need to submit scores that you can retake them if you need to.

Not registering far enough in advance as an engineering major to take the Fundamentals of Engineering exam before you graduate (If it's a requirement at your school).

Choosing a major in college based on misconceptions about what you will actually be studying and/or what work that will qualify you for.

Choosing a major in college based on the difficulty during college, and not the effects it will have on the rest of your life.

Choosing a major in college because your friends have chosen the same one.

Choosing a college because your friends have chosen the same one.

Wanting to go into a technical field in college, but not taking the highest math and science classes your high school offers, and being forced to make them up in college (Not very life-altering, but can set you back a year).