It’s called False Memory Syndrome.
From wikipedia: False memory syndrome (FMS) describes a condition in which a person’s identity and relationships are affected by memories that are factually incorrect but that they strongly believe. Peter J. Freyd originated the term, which the False Memory Syndrome Foundation (FMSF) subsequently popularized. The term is not recognized as a psychiatric illness in any of the medical manuals, such as the ICD-10 or the DSM-5; however, the principle that memories can be altered by outside influences is overwhelmingly accepted by scientists. False memories may be the result of recovered memory therapy, a term also defined by the FMSF in the early 1990s, which describes a range of therapy methods that are prone to creating confabulations.
https://en.wikipedia.org/wiki/False_memory_syndrome
I graduated in 1979, and there was a guy who went on to become a famous musician in the class that graduated in 1981. By our 30th reunion, just about everyone remembered going to backyard parties where he played during high school.
Not that they're lying, but I find it hard to believe that so many people saw him. If you look back at the yearbooks, he wasn't really all that popular.