I don't want to scare you, but i will tell you a story that is at the very bad end of the spectrum. This 4th year came in and talked to us about if we were having any problems to talk to a higher up immediately. Here is her story: she was out of the country during orientation week and didn't think it was a big deal. During the first week of gross anatomy and histo, etc, she was running around doing the things the rest of us did during orientation--TB test, immunizations, hosp ID and university ID made, filling out paperwork, etc. So she didn't have much time to study and when the first quiz in gross came around, she failed. The proceeded to end up failing gross and histo at the end of the 13 week block. It was all because it had snowballed so that she was trying to learn the old stuff and then add the new stuff. When the next block came around, she was so upset she started out doing horrible in biochem. The end of the story is good--they allowed her to make up things in the summer and she is graduating with her class and going into child psych and I think she'll be pretty good at it.
This is just the very bad thing that could happen. I'm sure others have a better story for you. I would assume it depends on what your school does during orienation week.