What should never have been invented?
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Though question. On first thought I would have said the Atomic Bomb. But how would the world be today? Better or worse. WWII would probably have ended differently. Later that's for sure. So how about something more simple like Pop-Up Ads and unskippable Ads?
Roundup- the chemical herbicide banned for being carcinogenic and the subject of many lawsuits.
Aristocracy
Just the idea that because you're born in a rich family means your better than other people is absurd
Automatic and semi automatic fire arms, and handguns. None of these are necessary for hunting, their only purpose is to kill people.
Leaded gasoline. Dude who invented it knew it was toxic and went ahead anyway with it because it was cheaper than other additives to boost octane. He also invented CFC's that destroy the ozone layer. Look them up and spit on their grave if you are in the area.