Let's go with the atomic bomb...if you disagree, consider that we made a weapon too powerful to ever be used again, but nations that have them get taken way more seriously in diplomacy.
And let's be serious, it's pretty much tick-tock, tick-tock before they get used again when they get put in the hands of zealots. Let's be doubly serious, it will be religion that convinces some leader that they are within their divine rights to cleanse the world of their enemies.
Human history, as a whole, is so depressing and meandering it's a weird question to try and answer. Were the great empires a success, or a failure? It depends on if you're measuring monuments built or social justice enacted, and if you're comparing against modern polities or whatever shitty local warlord they replaced. History doesn't really have an end goal, as much as we'd like it to.
Maybe you just meant a personal failure:
Thomas Midgley is one of my favourites, because he's famous for three things: Inventing leaded gasoline, inventing ozone-destroying PCBs, and inventing an accessibility contraption that strangled Thomas Midgley. He did nothing else of note; he's like the real life Bloody Stupid Johnson.
Pheidippides of Battle of Marathon fame is famous for running a long way just to deliver some news first, and then dying from exhaustion. People regularly make the same trip and are fine. He was regarded as a hero, and the races were originally in his honour, but I wouldn't want to be him. Edit: Maybe not a great example, actually. The story names a much longer distance than a marathon, although it's kinda mythical.
Muhammad II of Khwarazm received an envoy from Ghengis Khan, who wasn't bent on invading at all but wanted trade, and decided to steal their shit and kick them out instead. Then he killed the people sent next to ask for a nice apology. You can guess where that went.
The Soviets once tried to sextort Indonesian quasi-communist leader Sukarno with a tape. It did not work, because he was shamelessly proud of his "virility". In at least some tellings he misinterprets the KGB's presentation as a gift, although I doubt he could have been that dumb.
Ohh it's much worse than that. Usually humans would live to around 60 if they survived infancy before that. Their diet was varied and since food was a limited resource, there was no way of population blasts. But agriculture just fucked it all up. We stopped moving around since the land needed constant maintainence and since the diet became mostly carbohydrates, combined with back breaking work, our life expectancy dropped to 40. We didn't domesticate wheat, wheat domesticated us.
It took modern medicine... ie 20th century to get the average life expectancy up again.
I recommend you read the book called Sapiens. It's an eye opener.
Opting for gasoline over electricity early on when cars started to become a thing, we were already going electric, but a smear campaign put fear into people's minds about electric and switched tk gasoline.
Batteries could have been standard for a bit longer, but it seems to me that eventually the need to go faster for longer would have forced combustion engines to be a thing. All they had were lead-acid batteries (or primary cells, but that would be dumb) and new more energy-dense chemistries didn't show up for a long time after. Maybe they could have found one if they really needed, but it's a tricky science even today, so I'm skeptical.
It's possible, I suppose, that infrastructure could have been rolled out for both en mass, but I don't see an even mix lasting through the whole 20th century. Probably not even past WWII.
For me, this post is right under the person who said "Agriculture" and the response "Because it lead directly to feudalism and other forms of autocracy?"
And if unqualified businessmen ruling instead of experts in their "given fields" isn't a perfect way to describe feudalism, I don't know if irony has survived.
The election of Richard Nixon. I sincerely believe that's where we traded the "flying cars robot butlers" timeline for the "worst inequality of literally ever" timeline.
My friend and I argue this occasionally. The difference IMO is that Nixon was a politician while Regan was an ideologue in a politician suit. He wanted to push his agenda no matter the negative consequences. Nixon pushed the car down the hill, but Regan started it up and floored the accelerator.