Factual error: In a scene that takes place in 1956 or 1957, Alicia Nash places an orange Tupperware container in the refrigerator. Although Tupperware first became very popular in the mid-50s, the particular model of Tupperware used in the film was not introduced until the late 60s or early 70s.
Other mistake: When Nash is put into the car and is being taken away to the hospital, there is a line of people watching him drive away. Two of the spectators are the imaginary roommate and the little girl. They are standing in front of rest of the spectators. As the car is driving away you see a female student look around the imaginary roommate to see the moving car. If the roommate can only be seen by Nash why did she need to look around the roommate to watch the car drive away?
Trivia: The entire film was filmed in sequence. Meaning that each shot you see in the movie is the order in which the filmmakers filmed it. This is unusual for films as filming in sequence can be very inefficient, as the filmmakers have to keep revisiting the same sets and setting them up more than once rather than just filming all their scenes while they have the set ready for it.
Nash: In competitive behavior someone always loses.
Charles: Well, my niece knows that, John, and she's about this high.
Nash: See if I derive an equilibrium where prevalence is a non-singular event where nobody loses, can you imagine the effect that would have on conflict scenarios, arm negotiations...
Charles: When did you last eat?
Nash: ...currency exchange?
Charles: When did you last eat? You know, food.
Nash: You have no respect for cognitive reverie, you know that?
Charles: Yes. But pizza - now, pizza I have enormous respect for. And of course beer.
Nash: I have respect for beer. I HAVE RESPECT FOR BEER!