Sesame Street had so many great creative and musical segments before it turned into the Elmo show. It didn't talk down to children or insult their intelligence. It inspired children rather than just try to hypnotize them with colors and repetition to shut them up for an hour.
I love most of the old musical bits, but there was an animated short music video called "Carefully" that scarred me for life. In it, a kid is running down the hall and then a dracula pops out at him from a broom closet or something.
Of course, it was meant to be just one of a long list of situations in which one must proceed 'carefully,' but I was deathly afraid of vampires and closed doors for years.
I remember one where Bert and Ernie are exploring an Egyptian pyramid and the mummy comes to life... that scared me for the longest time. I guess the show has its own Nightmare Fuel page on TV Tropes for a reason[0].
Unfortunately, one episode where Margaret Hamilton reprises her role as the Wicked Witch of the West aired once and was banned after angry letters from parents complaining about how it terrified their kids. I'd like to see that one day.
I'm with you on the mummy one, that scared me too.
It always perplexed me though, why they thought the Count's lightning-thunder-and-bats routine [when he completed a count] was too scary and nixed it -- seemed pretty tame to me -- maybe the noise rattled kids?