Explaining low grades/low test scores
If you feel compelled to explain a bad semester/year or a reason why your standardized test scores are low, consider whether the personal statement is the best place to do it. If such an explanation can be smoothly integrated into the body of the essay, fine, if not, you might add an addendum in a different part of the application. Also, be objective and even-minded when explaining these areas. Graduate schools do not want to hear whining. They would prefer a mature and balanced approach.
Explaining your weaknesses
Be careful when graduate schools ask you to do this. Identify weaknesses that have already been able to overcome, or suggest weaknesses on which you can offer a positive slant. Don't reveal something that will sink your application.
Cardinal Sins, or "Never Do This When Writing a Personal Statement"
Never send a personal statement that contains grammatical or spelling errors or a personal statement that misidentifies the school for which you are applying. In an age of computers, students sometimes forget to change the name of the school. For example, you might write, "The English Department at the University of Massachusetts appeals to me because...", when you are actually sending the essay to the University of Connecticut.
Never over-exceed length limits. If the school asks for only two pages, two and a half pages might be okay, but four pages is not. On a similar note, don't change font and margin sizes to cram more material onto a page. Admissions personnel recognize this immediately and it annoys them.