Things that are widely believed, but not true

[Jarkko]

Real Native Americans were a highly diverse group and had distinctive languages, religions, and cultural practices for each region.

This. So much this. And in fact, not just distinctive languages, but numerous language families. British Columbia alone still has five language families (Salishan, Wakashan, Tsimshianic, Na-Dene (or Dene-Yeniseian if you buy into that theory), and Algic (if only for a small pocket of Saulteaux Anishinaabe spoken near where my aunt and cousins used to live in Hudson's Hope), plus two isolates (Haida and Ktunaxa) represented.

I suppose this flows into an expansion of what you posted. Not only did/do indigenous North Americas speak many different languages (perhaps into the thousands of these even though many are sadly moribund or at least endangered) but there are a whole host of families to which they belong. In Canada, for example, Algic languages run from Nova Scotia all the way over to northeastern British Columbia, Na-Dene is dominant in the Yukon and Northwest Territories (and Alaska as well) as well as much of inland northern and central BC, and Eskaleut dominates the Arctic. But there are also Siouxan languages, most notably in Saskatchewan (Dakota and Assiniboine) and Alberta (Stoney-Nakoda) but also some in southwestern Manitoba, Iroquoian languages in southern Ontario and southwestern Quebec (there are two Mohawk/Kanien'kéha reserves in the Montréal metro area), and an unclassified language spoken on Newfoundland called Beothuk.

Almost all of the language families attested in Canada are attested in the USA as well, the lone exception for now being Tsimshianic (at one time it was being sized up as a potential member of a larger Penutian language family, and there is still debate on this). The Siouan language family, in fact, is more present in the USA than it is in Canada, in the Great Plains States, and the most-spoken Na-Dene language, Navajo (or Dine bizaad) is spoken by more people in the USA than the number of speakers of every Canadian Na-Dene language put together. Other families that cover a lot of ground in the USA that aren't present in Canada include Uto-Aztecan - which is also still a major language family of Mexico with the Nahuatl languages having millions of speakers - in the southwestern/central-western USA, and Muskogean (originally) in the southeast. JHG mentioned the Comanche, and their language is Uto-Aztecan, as are Hopi, O'odham, Ute, and the Paiute languages (which aren't as closely related as their names would imply).

And then there's Mexico, which also has significant language diversity, with the Uto-Aztecan, Mayan, Mixe-Zoque, and Oto-Manguean languages families being the most widespread. There are several languages and possibly even a couple entire language families that have gone extinct within Mexican borders, and likely a few language isolates but not as many as there are held to be in the USA.

[Kyng]

Despite what many believe (and what the word's etymology would suggest), the 'equinox' is not when day and night are of equal length.

It is true that, on the day of the equinox, the centre of the sun is above the horizon for 12 hours, and below the horizon for 12 hours. However, there will be daylight as long as the top of the sun is above the horizon. Furthermore, because the Earth's atmosphere refracts sunlight, we'll get some daylight even when the Sun is entirely below the horizon!

Instead, the day on which we get 12 hours of daylight (and 12 hours of darkness) is known as the equilux. This occurs a few days before the Spring Equinox, and a few days after the Autumn Equinox. If you're still confused, the following short video explains it better:

[JHG]

The notion of a “progression” from stone to bronze, to iron, and ending with steel is oversimplified. Native Americans mostly used Stone Age tech before jumping directly to gunpowder(some even did use bronze at a time Westerners had long phased it out), Southern Europeans such as the Greeks used bronze in conjunction with iron, and Sub-Saharan Africans shifted directly to iron after stone.

Dang! Forgot Australians like the Wiradjuri. They basically used Stone Age tech and just went right to black powder.

[JHG]

Pirates burying treasure? Ye landlubberrr knows nothing! Arrrr! Why burrry trrreasurre when you can USE it? We pirrrates spend rrrr money on essentials like powderrrr, ammo, weapons, new clothes, rum, and frrresh orranges! Only Cap’n Kidd everrr burrried trrrreasurrre and that was embezzled from his bosses aka the British government. 
Shiver me timbers!

[JHG]

Your gun terminology has misfired! Oh boy...
No, not every gun capable of fully automatic fire is a machine gun. Machine guns are specifically crew served guns that fire rifle equivalent rounds(which mean automatic guns that fire pistol rounds are submachine guns or SMG) so the Gatling gun is no true machine gun. Assault rifles are rifles with intermediate cartridges. Battle rifles are the rifles with full size rifle rounds. Select fire rifles are rifles with multiple firing modes and selector switches. Automatic rifles are rifles with automatic fire(although the BAR has a misleading name as it's a light machine gun.) Handguns are just all handheld firearms(making rifles, muskets, carbines, and shotguns long guns.) Pistols are handguns with one chamber. Revolvers are handguns with a revolving cylinder. 
Now that you got all that, you're a straight shooter!

[JHG]

I dunno how this even needed to be addressed but statues are NOT a good way of remembering history. They're literal pedestals to hero worship the people they depict and the principals the people stood for. To remember history, watch a documentary, read a non-fiction book, or heck just listen better in history class. 
Also, also if you need to remember this better ask when's the last time you've seen a statue of Isoroku Yamamoto.

[JHG]

Whoever says Brontosaurus is just another Apatosaurus either grew up before 2015 or thinks the current consensus is too much. There was an analysis in 2015 splitting Brontosaurus off as a separate genus from Apatosaurus after about a hundred years of being a junior synonym and there is a debate on whether or not to keep them separate genera. Really though, they are similar sauropods due to being both apatosaurine diplodocids which can be distinguished by being bulkier than other diplodocids. They also wouldn't be totally identical even if Brontosaurus was a junior synonym because it would be in separate species. If lumping them together, they'd be A. ajax(type species), A. louisae, A./B. excelsus, A./B. parvus, and A./B. yahnahpin(which was once a unique genus called Eobrontosaurus just to be difficult.) Think of it this way: A red deer and elk are both members of the genus Cervus but separate species C. elaphus and C. canadensis so would you call THEM exactly the same? No. Heck, woolly mammoths and Columbian mammoths are both members of Mammuthus but they're obviously separate species.

[Jarkko]

John Calvin, in spite of agreeing with the notion that Michael Servetus should be executed for heresy, was ultimately not the one that had him burned at the stake. He and several Swiss Reformed pastors tried to have the sentence "lessened" to a beheading (which was less painful) but failed, as the council in Geneva that ultimately condemned him was headed by Ami Perrin (who would later try to remove John Calvin from Geneva entirely and massacre the French in the area while he was at it) and were basically contrarian to anything Calvin supported or didn't.

[JHG]

So...generals of the olden days(Ancient to Imperial Age at least before the proliferation of breechloading rifles and end of colorful, distinct uniforms) all fought with their men while today's generals lead from armchairs? Not quite. Men from lower and middle military ranks today still fight among their men before they get promoted to the higher ranks so...it's technically true that those with the rank general don't fight but other military leaders like captains do. As for Ancient-Imperial Age generals, it was more of an inconsistent take as while there are plenty of warrior kings like Alexander the Great, Carolus Rex, Richard Lionheart, and (I guess)George Washington there's also recorded individuals who didn't like Catherine the Great, Henry VIII(after his jousting accident), and Kublai Khan. The reasoning is identical to modern reasons why not to have rulers and generals fight alongside their troops; The whole chain of command is ruined if the top brass general, king, or president suddenly dies. Besides, some of those generals might be physically disabled or are really old and at any rate, high ranking generals, kings, and presidents are really busy people and they won't always have time to be good as the grunts.