For all eight seasons of Full House, actor-comedian Dave Coulier starred as Joey Gladstone, but despite being considered part of the Tanner family, he was never “Uncle Joey” like many viewers assumed him to be. The “Mandela effect” was coined by paranormal researcher Fiona Broome and refers to the concept of false memories being shared by a large group of people. There are many other examples of this in media as well, including the misspelling of “Berenstein” Bears. Joey appeared on both Full House and its sequel show Fuller House and has been misremembered for years as “Uncle Joey” despite evidence of the contrary.

https://screenrant.com/full-house-uncle-joey-mandela-effect-not-related-explained/