Focusing too much on the SAT score. If you have 1500 or above, stop re-taking it. While a lower score can filter you out, a higher score won’t help you in any way. Don’t forget, when it comes to HYPSM (Harvard, Yale, Princeton Stanford, and MIT, all the schools that admit less than 10% applicants), they reject 80% of students whose academic abilities are totally qualified.
Focusing ONLY on academics. Perfect academics only get you a review. Your essays, extracurriculars, awards, and recommendation letters get you in.
Applying to the wrong major.
Asking for too much financial aid from schools that do not need-blind.
Not applying for financial aid when you cannot really afford the $70 K price tag.
Applying to schools that have very few seats for internationals or special programs and being disappointed when they don’t get in. (MIT-87 this year, Cal Tech 30 seats this year, Upenn M&T 50 total globally)
Being rigid about conditions of acceptance. Many families don’t understand how admissions work and will worry that not being admitted to a specific major will be a problem or is a deficit in the long run. For some programs, yes. But, for the large majority, that’s not true. The truth is most students change major once at college. Few students end up with a degree in the subject they thought they would major in.
Trying too hard to impress the admission officers on essays, that after too many revisions, students do not feel anything and the essays lost their freshness and simply became not interesting.