"For some applicants there are anomalous factors or events that they might want to tell us about--that bout with mononucleosis during the sophomore year that caused their grades to go down one semester or their history of poor standardized testing. I would discourage applicants from using their personal statement to discuss such anomalies. Instead, applicants should prepare an addendum--a separate piece of paper that they attach to their personal statement, entitled "My Sophomore Year," for example, and write a few paragraphs to concentrate on that particular anomaly."
-Admissions Officer, University of California at Berkeley School of Law

Just about all schools will accept such an addendum, but you should call the admissions office to clarify the policy. What's certain is that no good can come from discussing negatives in your personal statement. That space should be reserved for making a positive argument for your candidacy. Another point you should keep in mind is whether you have a valid reason. Staying up late the night before the LSAT is not a legitimate reason for a bad performance, while documented sickness could be. A particularly bad semester could be explained by a death or illness in the family.

There are many more gray areas. For example, it is worth noting that you simply have a bad history of standardized testing? Doing so tactfully (in other words, don't rail against the arbitrariness of tests or demand the right to be considered for your grades alone) can help the schools understand your exact situation, but it most likely won't have a substantial effect on their perspective (they know to take into account the imprecision of standardized tests). What about the class for which you simply did not grasp the material, or a sub-par GPA during your freshman year? Again, what you have to say won't constitute an extenuating circumstance, since everyone has weaknesses and faces the same challenge of adjusting to college. Your best approach is to transform such blemishes into something positive by pointing out particular courses in which you performed well, especially those that were more advanced, more relevant to your intended career path, or more recent.

Finally, make sure that you do not take a contentious tone. Don't accuse your teachers of unfair grading standards or complain about a lack of extracurricular opportunities at your school. Be clear that you're not trying to excuse yourself of responsibility, but emphasize that you simply want the school to have the complete picture.