Cracks (which is also known as Crack Master) was an animated Sesame Street short that in recent history was believed to live only in the imaginations of those who were haunted by their childhood memories of it.
Since it had been long lost, those who had memories of it thought than maybe they had only imagined it and that it did not actually exist. That maybe they were the only ones living a persistent mental issue tormented by frightening false memories.
Until others started to recount it online. After all, it was terribly scary and did not seem like the kind of thing that Sesame Street would produce or subject young children to. One man said that decades later he still thought of it almost every day!
Then, it resurfaced in 2013. Now, you can enjoy the nightmare that is Cracks.