I know these two are NOT Sesame Street but I was (or yet, still am) afraid of:
TMS: Hugga Wugga. The eyes on Hugga are horrifying!
PS (Plaza Sesamo): I don't know what to call it, the animated bit where Abelardo, Pancho and Lola are gathered around a machine. Abelardo pulls a lever, a buzzer sounds and on comes a number on the screen. Lola does the same, while a different number appears, but the next time Pancho breaks the lever and the machine breaks with different colored cracks in the screen while Pancho is confused with why it broke. Abelardo waves and the gang leaves.