During the fight scene in the woods, Willem Dafoe actually cracked a rib when Robert Redford's stunt double rammed into him, tackling him to the ground during the shooting.
When Eileen returns home from the FBI and checks the mail, two identical looking business letters are visible that both have "Pittsburgh, PA" spelled without h in the recipient address. On the first letter the same mistake can also be seen in the sender address.
When Wayne is leaving the table by the pool, he picks up his jacket. In the next scene just a second later, he picks up the ball to the throw to the dog and his jacket is on. There is no way he would have had time to put on his jacket.
When Wayne is telling the story in the forest about his father dying the sticking plaster (put on from an earlier fall) is not there. A few scenes letter the sticking plaster is back on his eyebrow.
Eileen Hayes: You know, I never talked to him about it. I never ask him why. I just told him to get rid of her, and we got on with our life. I didn't want to know. I love him, and she admires him.