This is very easy to notice! In the beginning of Back to the Future, when Einstein (the Doc's dog) goes back in time, the trails of fire are left behind. If you look at Michael J. Fox's foot, it is clearly in the fire.
More specifically, the fire trails goes exactly inbetween the Doc and Marty's legs, but when it cuts to a different shot, the fire is not between either Marty's legs. It's more on the left or right side of him.
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