The battle between the Monitor and the Merrimac was one of the first things that came to mind. The battle ended in an inconclusive standoff, and made no real difference to the outcome of the Civil War.
I say this despite the fact that my Dad was a teacher at… John Ericsson Junior High School (“The Monitor School”) in Brooklyn.
But it made every wooden warship on the planet obsolete in a day and introduced the revolving gun turret.
If you consider the manned spaceflight program as a massive industrial subsidy in disguise, then it certainly wasn’t a complete waste. Quite a bit of useful technology was spun off from the space program.
You could argue that many of these things would have been developed eventually, at less cost, if there had been no manned space program. But it’s simply incorrect to say that “none of it has resulted in anything useful.”
In the same vein, I’d nominate the Hindenburg. Everybody knows about this; say, “Oh, the humanity!” and people instantly get the reference. The death toll was only 36, and it doesn’t even appear on most lists of aviation disasters. There was no discernible historical impact at all; airships were on the way out anyway. If the explosion hadn’t been captured on film, I don’t think people would still be talking about it now.
The Hindenburg is a good candidate.
It did result in the abrupt end of the dirigible as a means of long-distance travel, and it was AFAIK the first major disaster filmed as it happened.
Waterloo.
Sure, it was a big battle, but as far as Bonaparte was concerned it was too little, too late. The Empire fell at Leipzig - the Hundred Days were nothing more than a curtain call. Even if Napoleon had won, he probably wouldn’t have been able to hold out very long against the Seventh Coalition very long; and even if he did somehow manage to keep the throne, he would have been no more than the ruler of France. His reign as the terror of Europe was long over.
What about 9/11?
Not saying it wasn’t a huge event but honestly there were several Islamist attacks on the West prior to this time.
Someone was bound to say it, I guess. 9/11 isn’t a good candidate for this. Not because it hasn’t been hyped-up–it has–but it doesn’t quite reach the “most overrated event” category. Only reason I say this is because, while in terms of number of deaths the event is low on the list, the effect of the event is huge. I mean, look at all that has happened here because of it, and not because it is overrated, but because certain elements in government have taken advantage of it for their own purposes.
That isn’t a candidate IMO. The attack itself may not have been that great in the loss of initial lives but it did provoke a serious response from the U.S. and its allies. We wouldn’t have invaded Afghanistan or probably not Iraq if not for 9/11 for example. Chaos theory predicts that we would be a completely different world even domestically if 9/11 never happened. Overspending on the dual invasions was at least a large contributing factor in the economic crisises in the last 6 years. We don’t have a time machine to see what would happen if it never occurred but it would have undoubtedly been very different.
That is slightly different from other world changing sequences of events that results from one focal point. WWI was purportedly started because Archduke Franz Ferdinand was assassinated and that eventually led up to the killings of millions of people plus provided the seeds for Germany to start WWII. We will never know if that would have happened anyway because the whole situation was a powder keg on a gas stove. 9/11 wasn’t like that. The U.S. would not have invaded either Afghanistan of Iraq around that time if never occurred. Furthermore, the later Arab Spring that killed other dictators like Muammar Gaddafi probably would not have happened either.
I will vote the other way. 9/11 was a disproportionately influential mission in historical terms. It was mostly a failed mission that didn’t take out the White House and/or Capitol like they wanted to but the medium term consequences were much more extreme than the planners could have ever dreamed of (both and bad for their goals).
It was so visible, and its victims came from so many different backgrounds. It was an event with worldwide consequences.
Honestly, I figured the death toll would be in the 5 figures and possibly 6, and by the time everyone who was in the immediate area who will die from the aftereffects does, I may be right. 
Another vote for Charles Lindbergh’s flight. It’s one of those things that was bound to happen sometime, somebody had to be the first. I’m sure at the time it was quite incredible to think it could happen, but the adulation he received was quite disproportionate to the feat itself.
I don’t get the Titanic hate. It’s a compelling story and perhaps the reason that ships now carry enough lifeboats for everybody.
The JFK assassination is up there. One could make the case that without JFK at the helm during the Cuban Missile Crisis, none of us would be here. But other than that, his legacy is overrated and while his murder was sad, it wasn’t the history-changing moment that many think of it as.
England and Frances had already started moving toward ironclad battleships years before the American Civil War. The Monitor was a nice piece of work, but it represented the US playing catchup with the rest of the world’s navies, rather than a great leap forward.
I’m gonna go with the life and death of Princess Diana.
In related non-stories, the lives and histories of the British royal family for the last, er, few centuries?? How long has it been since any of them made a substantive decision about anything other than dress code, etiquette, or hats.
Right - and that latter is the only reason it’s remembered over other airship crashes and accidents. It’s an early example of a fenderbender being headline news because a news chopper caught footage of it happening.
It’s also a bit too recent to judge its long-term historical impact, imho.
I know somebody who hasn’t heard of Justin Bieber.
The American Civil War.
It was a civil war, not THE civil war.
Other places had bigger, longer & bloodier ones (not necessarily something to crow about)
The two sides were spoiling for a fight and if it hadn’t been over slavery, then greed and/or grievance triggers meant the conflict was probably unavoidable.
At the end of the day the primary achievement was that the Reunited States caught up with the rest of the civilised world on some of it’s own self-evident truths.
If the two sides come to some amicably negotiated settlement then by current times you could have three complementary and competing economic, democratic powerhouses in North America with innovation running way ahead of today and a reduction in world tension due to the absence of US isolationism/imperialism.
Alternatively there might now be a nuke-armed India/Pakistan style standoff over fundamentalist Christian ideology.
In the ship-sinking category, I’ll nominate the Lusitania. It sank in 1915. The USA entered the war in 1917.
Sank.