I've had 2 "bad" semesters, 3.0 and 3.1, and I got 2 acceptances. I'm in engineering school, so grades are a lot different than arts and sciences majors. I showed a positive trends of sorts; I had a 3.5 both semesters freshman year, but I wasn't taking any engineering courses. Then the real hard stuff kicked in soph. year, and my grades went down, then came back up. I think its perfectly fine to have one bad semester, as long as you have some explanation of why (illness, family problems, etc.) The 2 stats that matter most are overall GPA and MCAT. If you have a really competitive GPA, I doubt they'll can you for having one bad semester.