Many thought she'd died

Everyone knows Yoko Ono played an important role regarding the direction The Beatles took in the early 70's.

Some, however, claim to remember her dying of a drugs overdose not long after John Lennon was murdered, and are surprised to discover she's alive today (article written Nov 2017).

Most celebrity death Mandela Effects arise when someone actually dies and they are shocked at that point to realise they had been alive for all these years. The reports of Yoko's demise appear to be similarly greatly exaggerated. However, the people experiencing this are well aware she didn't die at this time but describe a dual memory, for which they remember both events even though this is logically impossible.

John Lennon's widow

Yoko was John Lennon's second wife, and known for her performance art, film making and unique vocal performances. She formed her own band, The Plastic Ono Band, in 1969 which featured Eric Clapton on guitar. In 1969 John changed his name by deed poll to John Ono Lennon. In 1973 the couple split up and John was living in New York with May Pang, a personal assistant. John's son, Sean, is also in the music business and collaborated with Yoko on several projects.

Yoko was next to John when he was murdered in 1980. In 1990, on the day which would have been Johns 50th birthday, Yoko arranged for a worldwide broadcast of their famous song Imagine. 

Every drop in the ocean counts.

-- Yoko Ono

There's a very confusing situation with people who remember hearing of someone's death, then freely admitting they lived after it. This dual memory of course applies to Nelson Mandela, who the effect is named after, since the people who remember his 1980's prison death know full well he went on to be the President of South Africa.