When Biff and gang are chasing Marty in the stairwell of Biff's Hotel in alternate 1985, Marty hops the staircase to avoid security.

If you watch his shadow, it disappears for a sec (while they pan away) and then reappears a few seconds later.

Looking at the lighting of the set, it looks like this shadow isn't his! I could be wrong, as it all depends on lighting and the shadow could actually be his, way up the stairs - which would explain the few seconds that the shadow isn't there. Unfortunately the shot cuts back up to show him hopping the stairwell again, and you cannot see if the shadow then moves.