The case
This is the case the phenomenon is named for. In 2009, writer Fiona Broome described discovering that she, and many others, distinctly remembered Mandela dying in prison decades earlier, complete with news coverage. She coined the "Mandela Effect" for shared false memories.
The record is unambiguous. Mandela walked out of Victor Verster Prison on February 11, 1990, in front of the world's cameras, led South Africa's first post-apartheid government, and died in December 2013. The '80s memory is the mind compressing a long, half-followed news story into an ending.
Why our brains do this
For kids in the '80s, Mandela was a name in the news and on concert stages, tied to a prison. A distant figure in a long confinement gets an imagined ending when the story goes quiet.
Sources: Wikipedia — Nelson Mandela · Wikipedia — False memory: the Mandela effect