If something is based on something that is not real, you can say it is illusory. Tales of seeing Elvis and Big Foot eating together at McDonalds are probably based on an illusory experience.

Although the adjective illusory can describe anything that's based on an illusion, it often has the negative connotation of being deliberately deceptive. Like a bogus investment scheme that seems to make money for investors: any real profits are totally illusory. Or the illusory claims that fad diets work — they only work while you're on the diet, and no one can survive on cabbage soup forever.

Definitions of illusory
  1. adjective
    based on or having the nature of an illusion
    “Secret activities offer presidents the alluring but often illusory promise that they can achieve foreign policy goals without the bothersome debate and open decision that are staples of democracy”
    synonyms: illusive
    lacking in reality or substance or genuineness; not corresponding to acknowledged facts or criteria
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