The adcoms biggest concerns in this area are:
1) did you finish the pre-reqs mandated by their school (i.e. organic, physics, bio, etc)
2) will you have your degree in hand by the time you matriculate?
3) your acadmic performance your last semester or two should not be significantly different from the work that they evaluated when deciding to let you in or not (i.e., the stuff listed on your AMCAS and transcript).
You absolutely should let them know if any of these elements does not obtain.
If you can retake the class next semester & it won't affect your graduating in time to matriculate, you might not need to worry.
If you are still worried, try contacting AMCAS to see what they expect you to do. (We were told to keep information current through AMCAS).
If you want to figure out what a typical med school thinks about such things, but don't want to tip your hand, contact a school that you DIDN'T apply to, and pose your question.