Around 6 years ago, I finally watched all of the special features on my Back to the Future DVD boxset. The next morning, as I was gushing to a co-worker about all the behind the scenes trivia I had learned she patiently nodded before laughing and politely asking, “How did you not already know that Eric Stoltz was the original Marty McFly? Even I knew that. I thought you were supposed to be the real film nerd, not me.” Ouch.
It was, as the AV Club recently put it, a kind of pop culture facepalm moment, i.e., when I realized I had taken too long to discover something which everyone already knew. The story goes that Michael J. Fox, the original choice to play Marty, couldn’t get out of his Family Ties commitments to be in Back to the Future. Eric Stoltz was cast instead, but after four weeks of filming the higher-ups (i.e., Spielberg, Bob Gale, Robert Zemeckis) decided he was entirely wrong for the role. He was cut loose. A deal was worked out with Fox where he would film Family Ties during the day and Back to the Future at night and on the weekends, and the world was the better for it (despite Fox’s understandably chronic exhaustion while filming).
To Back to the Future fanatics, actually seeing some of that original Eric Stoltz footage has been the holy grail. However, YouTube user Too Old to Grow Up just made a convincing argument that Eric Stoltz’ version of Marty has been right in front of us this whole time (or, more accuratley, right in front of Thomas F. Wilson when Marty punches Biff); we just never noticed it before.
We Minored in Film 
This was an interesting investigation. I am now wondering which scenes in “Aliens” still feature James Remar.
Note to self: Someone should do a list of actors who are still in movies which they were actually cut out off/recast. Eric Stoltz in Back to the Future, the stuntman who played Freddy for a little bit of Nightmare on Elm Street 2, maybe James Remar in Aliens, etc.