The airplane Hart makes from the envelope and the one that he throws from the rocks are two different planes.
When Jim falls on the ice, his position relative to the end of the sidewalk changes.
When Jim jumps on the bed next to Susan, his position next to her changes between shots.
(at around 38 mins) The geometric diagram seen on the blackboard behind Professor Kingsfield is the same one seen 19 minutes earlier.
When Ford first introduces himself to Hart in his dorm room, he's seen leaning against the right door jamb. In the next shot, he's leaning against the book shelves on the left.
During Kingsfield's cocktail party, Hart finds out his love interest is Kingsfield's daughter and is understandably perturbed. As she approaches him, he looks to walk out the door. At that point, Hart nearly runs into Thomas Craig Anderson but calls him 'Ed'.