i've heard that (at least at my school) there is quite a bit of flopping around.
now i'm sure it's not to the level of those who got straight As first semester are getting straight Cs next semester, or vice versa- when you're in one of those positions you're probably doing something exactly right or something wrong. but it seems like almost everyone has a lower grade at some point in their first year, and i've been told by quite a few people that their grades went up by almost an entire letter (ex B- --> A-).
EDIT: i wanted to add that i just found out last night that a friend of mine went from getting Ds and low Cs her first year, to almost straight As her second. granted, this is much different from making a similar improvement during your second semester (when grades are arguably much more important), but it shows it can be done.