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.
Right before, the scene of Alicia looking around the shed. We saw earlier John's memory of the shed, clean, with intelligence agents and electronic equipment. Now we see what it really looks like: somewhat dingy and falling apart, walls covered in magazine and newspaper clippings, and a simple home radio, no special equipment or people in sight. Its the clearest contrast between reality and John's imaginings seen the whole movie, and shows how bad the problem has gotten.
Nash protecting Alicia from Parcher, yet from her point of view, attacking her and knocking her down, with their child in her arms, no less. Her terror upon realizing that his schizophrenia has progressed to the point where he's a danger to her and their baby is palpable.
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.)
