'Unhistorical' fantasy tropes about medieval weapons.

1) Two-handed swords are the ultimate attack weapon.

This all started in the original D&D wargame where the true medieval weapon par excellence, the spear, only deals up to 6 points of damage per attack (1d6), while the two-handed sword deals up to a whopping 12 points of damage per swing (1d12 or in later editions 2d6).
If that stat had a historical basis, there would be no reason to use any other weapon in hand-to-hand warfare. Both ancient and medieval armies were dominated by spears, lances, and pikes. Swords were usually a one-handed backup sidearm.

To my knowledge, some medieval knights had a large 'Sword-of-War' attached to their saddle that could be used with two hands after the initial couched lance charge. Most scholars agree that these 'Swords-of-War' were not true two-handed swords, rather hand-and-a-half swords. The true two-handed sword, the German zweihänder or Scottish claymore, saw use primarily in infantry units of the 16th century to counter pike formations. In Scotland, where pike warfare had a deep and long history, claymores of various sizes were used from the 15th to the 18th centuries, though not all claymores were two-handed. Likewise, the Japanese nodachi / odachi and the Chinese miaodao are not prevalently attested before the 16th century. In all these cases, two-handed swords did not see popular use before the advent of gunpowder on the battlefield.

Therefore, my final judgment:
Fantasy warriors (like Conan the Barbarian) using two-handed swords in a world without gunpowder is 'unhistorical'.

2) Dual wielding primary weapons offers twice the offensive power.

In reality, dual wielding almost always required a smaller weapon, such as a knife or hatchet, to be carried in the off hand. I've seen LARP and other weapon enthusiasts try two primary weapons and they always end up hitting themselves. When your life is on the line, you cannot afford clumsy moves. In the chaos of a pitched battle, shields are extremely useful because they block attacks you cannot see well. To my knowledge, Tahir ibn Husayn Dhul-Yaminayn is the only historical figure who wielded two full-sized weapons in battle. A friend pointed out that Miyamoto Musashi was famous for fighting with two swords. All the reading I've done points to his use of one large sword (katana) with one smaller 'companion sword' (wakizashi).

Therefore, my final judgment:
Fantasy warriors using two primary weapons is 'unhistorical'.

3) Longbows were the medieval version of the machine gun.

Many English speaking fantasy fans cannot even imagine why anyone would bother using a crossbow because of its slower rate of fire. (Google it - you'll see this opinion is widespread in the English speaking world.) The longbow's supremacy, of course, is far overstated. Crossbows had a slower rate of fire, but the pavise offset this disadvantage.

During the Hundred Year's War, the English peasant longbowmen won a series of astounding victories over French armies. This gave rise to a sense of national pride that transformed these ordinary soldiers into superheroes. The average English longbowman gained powers to rival Marvel's Hawkeye or DC's Green Arrow. Sir Arthur Conan Doyle's fantasy novel about them, The White Company, is nationalist propaganda, not history. Although it borrows some names and events from the Hundred Year's War, the narrative itself is pure fiction.

In reality, the English longbowmen owed their brilliant victories to good soldiering, not to the longbow itself:
1) English peasant longbowmen were highly trained warriors who had very high morale. French peasants were poorly trained, dispirited, and disorganized.
2) The English crown pitted peasant longbowmen up against large numbers of French nobles. It was do or die - no quarter would be given if they lost.
3) The English archers used mauls effectively against heavily armored nobles in close combat.
4) The English commanders used the terrain brilliantly - hiding in forests, setting spiked stakes, using wagon forts, forcing cavalry and heavy infantry into muddy ground.
5) In all their worst defeats, the French knights scoffed at tactics, blindly charging into deadly traps and even trampling to death their own archers and crossbowmen.

Historians and academics go back and forth as to whether the English longbow was able to penetrate armor at that period or not. It must be noted that only one English source explicitly claims that the English longbow was able to penetrate armor at the Battle of Agincourt in AD 1415. All other English sources glorify the chaos and death endured among the French ranks without making any such a claim. All French sources uniformly deny that the English arrows were able to penetrate armor. Clearly, a propaganda war about the effectiveness of the English longbow stretched all the way back then.

At the Battle of Poitiers in AD 1356, a French chronicler claims the dismounted French men-at-arms, "protecting their bodies with joined shields, turned their faces away from the missiles. So the archers emptied their quivers in vain." The real danger posed by the English longbowmen's arrows was getting hit in the face or getting thrown from an unarmored horse.

To settle the issue, please consider:
1) After the Battle of Crecy in AD 1346, the French knights began dismounting and approaching English longbowmen on foot. If the English arrows could pierce their armor, why would they prefer to WALK towards thousands of arrows?
2) Historical reconstructions generally prove that the cloth gambeson alone would have been effective at stopping most arrows from an English longbow. French knights wore steel mail haubergeon and the plate armor on top of thick cloth and/or leather gambesons. No simulation has shown an English longbow, even at maximum possible draw weights, capable of piercing all three.
3) At Battle of Pontvallain in AD 1370, the French equipped their horses with barding (horse armor). The accounts say that this rendered the English longbowmen ineffective and they were annihilated.

Irrefutable proof that the longbow was only a minor factor in the victories of the English longbowmen comes from history itself. At the Battle of Patay AD 1429 the English used the same tactics that won them their amazing victories at Crécy, Poitiers, and Agincourt. Under Saint Joan of Arc, the French peasants' morale improved immensely. The French cavalry had learned to respect the foe and employ tactics. This time around, the French commanders did not rush blindly into the English ambush and the French horsemen did not charge into spiked stakes or wagon forts. 1,500 French men-at-arms wheeled around to the flanks and killed or captured half of the 5,000 English longbowmen waiting for them at the cost of less than 100 French soldiers.

Did the longbow's rate of fire suddenly decrease?
Did the longbow suddenly lose is powers of penetration?
No. The French simply became competent soldiers.
With a more battle savy generation of Frenchmen, the English longbowmen were never able to replicate their victories. But their impact on the English imagination was so vivid that England continued to employ longbowmen in their armies well into the Renaissance, even after it was widely accepted that the musket was far more effective.

It was the MYTH of the longbowman that was England's most powerful weapon during the Hundred Years' War. Centuries later, the RAF pilot became the next English superhero. The Nazis planned to demoralize England by bombing their cities. The MYTH of the invincible RAF pilot defending the skies, even as the bombs fell killing over 20,000 Londoners and making millions homeless, kept England's morale high.

Therefore, my final judgment:
The fantasy longbow's rapid-fire, armor-penetrating superpowers are 'unhistorical'.