When the cannon breaks loose and races down a hill, it hits a ditch and bounces out coming to rest on the other side. In the next shot, the captain is inspecting the damage as the cannon rests squarely in the middle of the ditch that we've just seen it bounce out of.
When the cannon is being floated over a river on a raft, one of the four drag lines breaks, allowing the gun to charge down the river through rapids, eventually crashing on a rock. When the raft is shooting the rapids, however, all four lines are trailed out on either side, including the "broken" one.
Juana's top shows tears on the left sleeve and in the left armpit prior to the attack on Avila. However, after the attack, there are no tears in her top at all.
Miguel and the peasants push balls of burning straw down a hill into the French camp. One goes into a wagon of gunpowder which blows up the wagon but it explodes before the straw ball gets to it. Confirmed by slow motion or freeze framing.
During the attack on the military camp in the mountains, the guerrilleros jump on a burning wagon with powder barrels in order to take it with them. The number of barrels on the wagon changes in between shots.
The French commander at Avila, after sentencing 10 peasants to death, supposedly signed multiple documents with the same quill pen without dipping it into the inkwell frequently as must be done with quill pens.
During the attack on the military camp in the mountains, the flaming balls of dried grass roll down the mountainside into two different gunpowder caches which explode instantaneously. The fireballs would at least have to burn for a time to burn up the containers storing the gunpowder before it could explode. And no military stored it carelessly so that the slightest spark would set it off, it's too valuable for their efforts and is handled and stored with extreme caution.
After the wall has been breached, the commander of the French Garrison orders the Captain to "Mass your rifles in the square." The French army did not use rifles in the Peninsular war as Napoleon had discarded them as being too slow to reload.
The French commander at Avila threatened local peasants to tell him where the cannon was shortly after the journey to Avila with the cannon started, but Avila was far away from the cannon and the cannon was heading toward Avila and the peasants couldn't have even known where it was or even known about it. Word did not travel that fast in those days.
The French have built a bridge on boats over a river. Anthony and Jose put two barrels of gunpowder linked to fuses into each boat. Then when a troop of French soldiers are on the bridge, the fuse is lit at either end. After the explosion although the soldiers are lying, supposedly dead much of the bridge is still intact whereas it should have been blown to bits.
In the night scene when they are camped amidst the windmills, the smoke from the fires just goes up and hangs there but the windmills are spinning madly the entire time.
When the Spanish army is retreating at the beginning, nearly every close-up showing a musket reveals that none of them have triggers, clearly indicating that they are solid wooden mock-ups.
After the cannon has tumbled down the hillside, it comes to rest in a ditch. The Englishman inspects it and proclaims that the barrel has been separated from the carriage. But in plain view, the cannon has almost broken in two since it is not of metal.
During the knife fight scene with Carlos, Anthony (Cary Grant) holds his knife backwards, with the sharp edge facing himself instead of toward Carlos. Carlos holds his correctly, with the sharp edge facing Anthony.
When Juana is bathing in the River, she can clearly be seen to be wearing a swimsuit.
Juana refers to Anthony as "Duke of Wellington." This is in 1810. Sir Arthur Wellesley was elevated to the Peerage after the Battle of Talavera and to a Dukedom in 1814. In 1810, he was still Sir Arthur. The post of Duke of Wellington did not exist.
When Anthony yells at Miquel to jump, his mouth is saying something else.
When Anthony (Cary Grant) and Miguel (Frank Sinatra) jump off the cannon as it's rolling down the hill, their stunt doubles jump onto ground and it rebounds up into the air all around them and shows to be a net. Clearly, there is a spring-loaded catch net in the ground to soften their fall. And they don't roll or tumble, just hit the ground and bounce and stop, even though the cannon is rolling pretty fast.
When the peasants attack the military camp in the mountains, a peasant throws a firebrand onto a tent which rapidly catches fire. But as he is throwing the firebrand, the tent is clearly visible with a pattern of liquid that was splashed or poured onto it from the top down, and that part of the tent that was wet is the part that rapidly catches fire, showing it to be an inflammatory agent such as fuel oil or kerosene.
When Anthony and Carlos have their knife fight, the shooting angle from the top of the windmill clearly shows the shadow of the camera on top of it. It's even more obvious when the windmill arms pass in front of the camera, and the shadow takes up almost half the screen.
The trip of the cannon throughout Spain is a monumental laugh to any Spaniard watching this movie with a minimal knowledge of their geography. The story begins in Santiago de Compostela, in North-West Spain. The cannon has to go to Avila, in Castile, not very far from Madrid (about fifty miles North-West of the capital). Yet the way it gets there is first through The Haunted City of Cuenca (South-East of Madrid) and then the village of Campo de Criptana in La Mancha (South of Madrid)... This is like, say, a trip from London to Bristol via the Scottish Highlands and the Lake District. Or a trip from New York to Chicago and you pass through the Monument Valley and Mount Rushmore.
On the wall of the French army general staff there is a map of the region of "Las Cruces" (actually Galicia, northwest Spain). The river passing through Orense city is marked like Rio Duero. In fact it is Miño river which passes through Orense and delineates a section of the border between Spain and Portugal. The Duero river is further south, crosses the Castilian plateau and has its mouth at the Portuguese city of Porto. In the film it is named Rio Cano.