Other mistake: At the end, during the neighborhood fight scene, Monty is punching Joe, but you can clearly see that he is not actually coming in contact with Joe's face.
Continuity mistake: When Monty has just dropped off his kids from the fish aquarium, he and Julia are sitting in his car and she starts to cry and they talk. At the end of the talk, there is a tear on her right cheek when Monty kisses her, but in the shot from behind Monty a split second later, you can see her cheek and it is dry.
Plot hole: How could it be that Monty could possibly know exactly that Joe and the ex-wife were driving and where they would be to time that collision perfectly? A bit odd in reality to time a plan like that.