During the Making of Back To The Future (1), Michael J Fox was not the original actor to play Marty McFly. Apparently more than half of the movie was made with Eric Stolz (I think) playing Marty McFly.

However, and for what ever reason - I don't know - (but am glad), Michael J Fox was taken on board to play Marty. Due to the already large amount already filmed - the makers decided to re-shoot only the parts that Marty was visible in. THEREFORE!!!:

When Marty is in the Cafe in 1955, scenes with Marty in the shot have different "other customers" or "extras" than the scenes without Marty.

Look Close!!!