According to Snopes, the Mandela effect is a collective mis-remembering of a fact or event and one of the most notable examples is The Berenstain Bears. Droves of people remember the iconic characters as The Berenstein Bears yet despite online rumors, the Bears were always Berenstain—pronounced Bear-en-stain, NOT Bern-steen.
The Mandela Effect appears to have skewed nostalgia, and, thanks to the Internet, the effect appears to be collective. David Emery thoroughly explained this phenomenon in his July 24, 2016 Snopes article.
“No single example of the Mandela Effect has generated more online buzz than that of the children’s book series and animated TV show The Berenstain Bears. Quite a few people who grew up with the series, it turns out, remember the title being The Berenstein Bears, with the name ending in ‘ein’ instead of ‘ain.'”