The baby that almost drowns in a bathtub. Alone. And crying for mommy as the water pours into its mouth. Though the baby was rescued, it's still extremely haunting, especially to a parent.
Basically, the whole concept of the movie is chilling: a genius who is also a paranoid schizophrenic but almost never realizes it until somebody points it out to him. Imagine suddenly learning that someone like a good friend of yours is nothing but a figment of your imagination, all while interacting with them up to that point as if they were as real as everyone else. (Granted, this is technically artistic license since Nash only heard his hallucinations, but the movie's portrayal of this idea is still nightmarish.)