When was it last used?

Along with the rack, thumbscrews and burning at the stake, people associate the guillotine with medieval punishment and execution methods. Most famously, the guillotine was used in the French Revolution to dispose of that countries monarchy. So when most people are asked to date the last time it was officially used, they come up with years in the 1700 - 1900 range, with a few being aware it was used during the second world war.

The actual year of 1977 comes a such a shock that it's being classed as an MMDE, because the feeling is this is exactly the kind of thing which should be more widely known.

Last public guillotine execution video - 1939

Eugen Weidman was the last public victim of the guillotine - from then until 1977, it was all done behind closed doors. It was the official method of capital punishment until 1981.

Some Mandela Effects are pointed to by those who experience them as evidence of time travel, because, they argue, some events ought to be much more widely known that they actually are, and that is exactly how things would appear if something were suddenly inserted in the past of a timeline where it wasn't present before.