Interesting phenomenon. Basically a feeling of unease you get when something happens that you swear had already happened (similar to deja vu), or something that you thought was true which turns out it isn't.

Named after Mandela, since there were a lot of people who swore he could've died in the 1980's.

I genuinely felt this when Ronnie Biggs died, was confused as I could've swore I remembered him passing about 5 or so years ago.

Other instances include people being certain New Zealand is to the West of Australia, and that there's a portrait of Henry VIII holding a turkey leg.

Lots of strange metaphysical explanations, alternate timelines etc.