Frightening Sesame Street short

My father remembers watching a short skit on sesame Street which terrified him. From his account, it featured a black screen, with what appeared to be somebody pointing their finger at the viewer. With a voice, beginning softly, repeating the word "you." Progressively growing in intensity before it becomes a screaming "You!" Pointed at the viewer. My father had made reference to this short scaring him many times, and even imitates the screaming voice he heard. He was born in 1954, and must have seen this short in the late 60's or early 70's, during the very early years of Sesame Street. If anybody remembers, it'd be nice to hear of it.