10. Use poor spelling or poor grammar _ This one should be pretty obvious. Academics on admissions committees are generally high achievers with high standards who won't disregard even simple typos. If your personal statement is not technically perfect, it can make you seem sloppy, lazy, or inattentive, which are not qualities anyone wants in a future colleague. Remember that the people skimming your essay are seeking a reason to reject your application and make the pile of possible admits smaller. Always get someone with strong writing skills to review your essay.
As mentioned, this part of your application can be used to decide whether a school wants to invite you for an interview or not. Schools only accept students who have been invited for interviews, so getting to this step is a huge part of the process.