1914: Archduke Franz Ferdinand's driver accidentally starts down the wrong road while driving his coach about. He realizes his mistake, but has to back the coach up in order to get onto the right path. While the coach is doing this backing and forthing, a young Serbian yob by the name of Gavrillo Princip suddenly finds himself with a use for that gun he was trying to figure out how to get rid of...

Wow that really happened. And the whole conspiracy is probably the last time Bosnian Serbs and Muslims cooperated. Imagine fin-de-siecle Little Green Footballs if the first assassin-wannabe, a Muslim with the name Muhammad, had shot the Archduke.

Not sure it's a blooper, though, as it turned out successful. Sort of the reverse.

Perhaps when the mongols were poised to take the rest of Europe and the Middle East, and the whole thing had to be cancelled because the khan back home died.

Eh, not really a blooper. I think historians weeded out the bloopers, artistic license and all. Wracking my brain here, maybe I'll make something up.

The blooper was on the part of the archduke's driver, whose wrong turn made it all possible.

I've always rather liked the battle of Tondibi in 1591.

Small Moroccan expeditionary force crosses the Sahara and comes up against the might of the decaying, but still formidable Songhai state. The Songhai, with a ~ 10/1 numerical advantage decide to use a traditional West African tactic and stampede a heard of calvary at the Moroccan center, covering an advance by their infantry. But the Moroccans have imported firearms - a single volley and the cattle panic and immediately stampede back in the other direction, right through the Songhai lines. The shock of that plus the novelty of being shot at causes the entire Songhai army to route in panic and the Songhai empire functionally comes to an end.

Klaus was definitely on the right track.

In 1218, the Shah of the empire of Khwarizm -- today's Iran plus a sizeable chunk of Central Asia -- was involved in a minuscule trade dispute (about a caravan seized by a local governor) with the Mongols of Djinghis Khan, with whom he had had otherwise peaceful relations. Rather than negotiate this triviality, and either purchase or release the traders, the Shah decided to torture and kill the Khan's ambassadors and send their shaved and decapitated heads back as his answer. This would prove unwise.

As Wikipedia puts it: "more than half of Persia's population were killed, turning the streets of Persian cities like Neishabur into "rivers of blood", as the severed heads of men, women, and children were "neatly stacked into carefully constructed pyramids around which the carcasses of the city's dogs and cats were placed". In a letter to King Louis IX of France, Holaku, one of the Genghis Khan's grandsons, alone took responsibility for 200,000 deaths in his raids of Persia and the Caliphate. [...] The waves of devastation prevented many cities such as Neishabur from reaching their pre-invasion population levels until the twentieth century, eight centuries later."

On the more rhetorical level, or ironic one, there was the actual dialogue just moments before Lee Harvey Oswald (acting alone, folks) fired the 3 shots that changed history.

Gov. Conally of Texas, in limo: Well, Mr. President, you can't say Texas doesn't love you.
Pres. Kennedy: No, you sure can't.

On Hulagu's sack of Baghdad:

Many accounts say that the caliph failed to prepare for the onslaught; he neither gathered armies nor strengthened the walls of Baghdad. In fact, he had done the very worst things he could have done, he had angered Hulagu, given him an excuse to sack Bagdad, and done nothing to prevent the tragedy.

I mean, I can't blame ol Hulagu. They kept teasing him, first Persia, then the Abassids. Pretty much asked for it:

The Mongols looted and then destroyed. Mosques, palaces, libraries, hospitals — grand buildings that had been the work of generations were burned to the ground. The caliph was captured and forced to watch as his citizens were murdered and his treasury plundered. The caliph was trampled to death. Marco Polo reports that Hulagu starved the caliph to death, but there is no corroborating evidence for that. Most historians believe the Mongol accounts (and Muslim) that the Mongols rolled the caliph up in a rug, and rode their horses over him, as they believed that the earth was offended if touched by royal blood. All of his sons but one were killed. Prior to this, the Mongols destroyed a city only if it had resisted them. Cities that capitulated at the first demand for surrender could usually expect to be spared. Cities that surrendered after a short fight, such as this, normally could expect a sack, but not complete devastation. The utter ferocity of the rape of Baghdad is the worst example of Mongol excess known. (It is said some Chinese cities suffered a similar fate, but this is not documented).

Baghdad was a depopulated, ruined city for several centuries and only gradually recovered something of its former glory. Of all the Mongol Khans, he is, for obvious reasons, the most feared and despised.

Many accounts say that the caliph failed to prepare for the onslaught; he neither gathered armies nor strengthened the walls of Baghdad.

Australia's shaykh Hilali could put this one in his sermon. Better than the skit about the cat and the uncovered meat.

Pride of place has got to go to Major General John Sedgwick. While harranguing his soldiers at the beginning of the battle of Spotsylvania, Sedgwick shouted, "I'm ashamed of you, dodging that way. They couldn't hit an elephant at this distance!" General Sedgwick rounded off his remarks by immediately taking a bullet to the head and earning himself a treasured place in the annals of total fuck-ups. As a sort of posthumous icing on the cake, General Sedgwick turned out to be the highest Union casualty of the entire U.S. Civil War.

http://www.civilwarhome.com/sedgwickdeath.htm

That's along the lines of a classic blooper. Reagan's outlawing the Soviet Union outtake is a possible one.