Marty's girlfriend in Back To The Future was played by a different actress--and the reason why is pretty sad.

As any production, Back To The Future II had to change the actress who played Jennifer due to personal problems.

Claudia Wells played Marty McFly's girlfriend Jennifer Parker in "Back to the Future." Due to her mother falling ill, Wells could not return for the second film and was replaced by Elisabeth Shue in "Back to the Future: Part II."

Due to the change in actresses, the final scene of "Back to the Future," which was used in the beginning of the second film, had to be re-shot with Shue.

(Source)