During the 12th century the Knights Templar used the Maltese cross on their habit. It wasn't until a century or so later they were forced to change to the Roman cross. Unfortunately, this is a mistake that Hollywood has always made.
In the movie Jerusalem is shown to be in a flat, sandy desert. Jerusalem is actually set on top of a mountain (Mount Zion) with massive valleys filled with trees, and other greenery surrounding the city walls.
In the movie we see Mullah (Khaled Nabawy) wearing clothes with inscriptions of the Quran on the arms. In Islam, it's completely forbidden to write any of the names of Allah, or any verses from the Quran on clothes. Although it is allowed to write "Allah is the great" or "there is no God except Allah and Muhammad is his messenger" on flags.
The so-called 'Templar' who attacks Balian before the battle of Hattin (in 1187) wears a white surcoat bearing a black cross: the arms of the Order of St Mary of the Germans (aka the Teutonic Knights). This order was not founded until 1190 at the very earliest.
When the Muslims are praying near Jerusalem they are praying towards the setting sun, west, not towards Mecca which is to the south south east.
Contrary to what is stated (that there were no knights in the city), Jerusalem did have a modest number of knights and men at arms, but not enough for a proper defense. Sixty new knights were created from squires and freemen (not the huge number shown in the film, and not from random servants). These provided the main defense of the city. Although their help was requested, almost no people from the population of the city joined in the defense.
In the Director's Cut, Sibylla tells her son, soon to be Baldwin V, in his geography lesson, that the King of England is Richard, the son of King Henry. Richard I did not succeed his father until 1189, three years after the death of Baldwin V.
Contrary to what is shown, the casualties among the defenders in Jerusalem were actually only a few each day until the final fight. This was typical for close fought city or castle defenses, and reflected the extreme advantage of fighting from thick high walls with well placed towers.
While Balian is in Sicily, some crusaders can be seen carrying a banner with the arms of Castile and Leon quartered. However, in the 1180s Castile and Leon were separate kingdoms; it wasn't until 1230 when they were reunited and the quartered banner adopted.
The effects of trebuchets have been extremely overdone. They neither reach as far as the film implies (certainly not with that kind of angle) nor does the impact of the ammunition cause the ground to tremble as if it were modern artillery. A hurled stone on a massive city wall may cause a dent and merely a thump upon impact.
During the ambush on the road in France, archers are shown using bows which are at least five and possibly six feet long. Such weapons would not be used in Europe for more than one hundred years, and even then they were only common on the British Isles. The bow in use at the time was shorter and had a relatively low pull weight, which was of very limited use against a well armored target. Further, in France, the bow was considered a weapon of war, and its use was prohibited by all but the ruling elite (and then only for hunting since the bow was not an honorable weapon), so archers (vice crossbowmen) were very rare.
Early in the film when performing his blacksmith duties, Balian is shown hot shoeing a horse with a forged iron shoe. However, iron horseshoes were not in widespread use in France until at least the 13th century, and hot shoeing didn't come into practice until the 17th century.
Contrary to what is shown, at the time of the Siege of Jerusalem, Balian was married with two children. He was in Jerusalem intending to take his wife and children out of the city.
The trebuchets are shown with metal axles for the throwing arm (this is most visible as the camera moves past a trebuchet before Balian's knighting speech in Jerusalem). The technology of the time did not permit the creation of such large pieces of metal. Throughout medieval times a trebuchet axle would have been made from a thick timber.
Throughout the siege of Jerusalem, when the defenders use projectile weapons such as bows, trebuchets, crossbows, etc., the command given to release the projectile is "Fire!". This command did not come into use until the advent of matchlock firearms in the mid-15th century. The correct command would have been "Loose!".
When Balian is building the timber water channel he places the lower level duct on top of the higher one. This should be the opposite way round as in its current state any water running down would run under the lower duct and consequently be lost.
The way for the final assault on the walls of Jerusalem was prepared by a variety of siege engines (although not trebuchets). However, the section of wall shown turned into rubble could not have been destroyed solely by siege projectiles, even over a number of days. The wall was actually brought down by a mine (a short tunnel under the wall).
Throughout the battle scenes, the crescent moon and star are used as symbols of Islam. However, these were only adopted by Muslims in the 14th century, beginning in North Africa. 12th century Arab armies would have carried black, green or white flags.
Trebuchets throw their projectiles in a notably high arc, not the low direct path shown.
During the sea voyage and at other times, Balian is costumed in a top cinched with a row of buttons, almost a century before functional buttons appeared in his part of the world. Nonfunctional decorative buttons go back far in history, but functional buttons first appeared in the 1200's in Germany, spreading to other parts of Europe in the later 1200's through mid-1300's. Using tie strings or some manner cinching laces (or better, just a loose pullover top) would have been more period-appropriate.
The kind of helmet worn by the Templar that tries to kill Balian wasn't used until the next century.
A few times during the movie, the Muslims are shown praying while the prayer call is being delivered. The prayer call precedes the prayer.
All the shields were produced from the same moulds and are all the same, right down to the dents and arrow holes.
After the siege "all" were not "safely escorted to the sea". Rather, a ransom was paid, which allowed somewhat less than half of the actual refugees to go free. Those too poor to pay a ransom were taken into slavery.
At the beginning of the film, when the party is ambushed, a rider charges towards Godfrey. He is wearing a conical helmet, but in the next shot, after Godfrey has dispatched him, his helmet is flat.
Balian's arm was slashed by a sword perpendicular to his arm, however, the injury shown later has the gash going length-wise down his wrist.
Saladin's recitation of Al-Fatiha (the first chapter of the Holy Quran) is not complete. After the surrender of Jerusalem, we see Saladin reciting the Sora, but he does not finish before he starts another one.
In some scenes involving horses, modern orange cones can be seen on the ground directing the riders on the path past the camera to follow. This has been corrected for the Director's Cut DVD.
Guy stabs Saladin's messenger in the throat with a small dagger. When the messenger falls, Guy is holding a bloody sword, rather than his dagger. This is corrected in the Director's cut, where Guy proceeds to behead the messenger with his sword.
Patriarch Eraclius is depicted as a coward who advocated abandoning Jerusalem and its inhabitants in order to save his and Balian's lives. In actuality, Balian wanted to leave the city, as he had come only to evacuate his wife and children and had sworn an oath to never take up arms against Saladin in order to cross the siege lines. Eraclius absolved him of his oath and encouraged him to fight Saladin long enough to negotiate a truce for the lives and freedom of Jerusalem's citizens. He then had all the churches' silver stripped down and melted into coins to pay the defenders. After the siege, Eraclius and Balian collected money from the city's wealthy to pay ransom for 18,000 inhabitants, and offered themselves (in vain) as hostages for the remaining 15,000 who couldn't pay.
The size of the hole made by the sword in the sand when Imad falls after the fight for the horse changes; from large and uneven to even
When the messenger from Saladin is stabbed in the throat by Guy de Lusignan, the blood that is squirted outward sprays from the left side of the neck where he was not stabbed.
In the Kerak battle sequence, the lighting changes back and forth drastically between cuts, from harsh midday light, to soft morning or sunset lighting.
When the priest's assistants are burying Balian's wife, the priest instructs them to behead the corpse. One of them picks up the axe but in the close-up of the corpse's face, the axe is still lying on the ground next to her.
During the battle for Jerusalem, the crescent moon and the star in the banner change positions during a short period of time. First, the crescent is horizontal, with the star a short distance above it. In the next scene, the crescent is on an angle, and the star is where the unlit part of the moon would obscure it.
The movie shows Muslim martyrs (shaheeds) enshrouded. A normal deceased is enshrouded while a martyr is not.
In the film, King Baldwin IV (Edward Norton) wears an iron mask to hide the horrific signs of his leprosy, but the real Baldwin IV "refused to hide himself as king even as the disease consumed him".
The castle of Raynald of Chatillon, called simply "Karak" in the movie and known as "Crac des Moabites" at the time, is shown on a modest rise above a plain. It actually sits atop a ridge surrounded by fairly steep valleys.
During the second day of the siege, Balian watches several Saracen soldiers put their flag upon one of the towers; Balian is seen taking a weapon from a nearby archer, if you look closely that weapon is a war hammer. In the next shot, Balian is seen slashing a Saracen with an arrow.
In front of Reynald's castle, when Saladin's army comes, a yellow plastic pole that marks a stopping point for the foot soldiers is clearly visible. (This has been corrected for the Director's Cut DVD.)
In the directors cut version, Balian is in Jerusalem, praying where Christ was crucified - you see him burying his dead wife's silver necklace (cross). Later in the movie, you see him pulling out the ring that Sibylla gave him and kissing it - not the cross of his late wife.
Balian's hair keeps changing between shots when he and Tiberias talk on the battlefield.
Prop weapon visible, when the English Sergeant executes the son of Roger de Cormiere, his pick axe weapon clearly has a silver blade, when he removes it from Cormiere's skull, the blade is no longer on the weapon (nor is it in Cormiere's head).
After the breach in the wall during the siege on Jerusalem, when the camera pans overhead there is no obvious evidence of rubble despite a relatively large gap.
In the beginning of the movie when Balian is pounding on his piece, he is shown quenching the end of the piece after pounding on it, with the middle of the piece still hot enough to work. However, the next shot shows him once again working on the hot end, while the middle is cool.
(at around 2h 5 Mins) When Balian speaks to the people of Jerusalem after surrendering the city, a lock of hair blown across his face vanishes mid-frame.
From 36:05 on, for some seconds, behind Orlando Bloom, in the distance and out of focus, a light moves rapidly, resembling the headlights of a modern vehicle.
In the beginning of the film the blacksmith is hammering a hot piece of iron which he puts in water and immediately starts to hammer again. The iron is red hot even after being dropped in water.
At about the 1 hour 50 mark during the siege of Jerusalem the crescent moon can be seen above Jerusalem as the camera pans back from Balian. This is incorrect as the moon went from waxing gibbous to waning gibbous during the dates of the siege (20th September, 1187 - 2nd October, 1187).