When the old Biff is giving Marty the almanac in 1985, there's no rear view mirror on the front wind shield of the car they are in. Later on, when the young Biff is driving the car, you can clearly notice someone has installed one. ;)
Actually, this is not really a slip up, but more of a production trick. In almost no movie where an insert vehicle is used will you find a rear-view mirror OR headrests on the back of the front seats. These are removed for better views of vehicle interiors as well as talent shots. You've got a good eye, nonetheless...
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