When the Amigo's believe they are performing a show for the towns people, Lucky gets shot in the left arm with a real bullet. You can clearly see blood on his costume, but in all the scenes to follow there is no blood.
Ned's chrome-plated pistol jumps from being stuffed down the front of his pants to his holster between cuts after the gun battle.
When Ned Nederlander is swinging on the ropes, he gets his boots
stuck in a piñata. At first his boot tips are going to get stuck as he is facing the sky. The next scene he his boot heels are stuck in the piñata and he is facing the ground.
When Martin Short is about to have a duel with the German, at first Chevy Chase is looking on wearing clothes like the outlaws' then he is suddenly wearing the Three Amigos uniform without any time or opportunity to change.
When the Amigos approach the Singing Bush, the instructions Dusty reads for summoning the Invisible Swordsman require each of them to fire a shot in the air and say a magic phrase. Instead, they perform the ritual in reverse, saying the magic phrase first and then firing a shot.
After Steve Martin escapes the cell at El Guapo's and takes a position on the wall overlooking the courtyard, you can see broadcast antennas in the background on the mountains.
Any instances of badly synced musical numbers, fake-looking sets, and gaps in plot logic can be chalked up to the fact that the film itself is paying homage to cheaply-produced B-movies featuring washed-up actors.
At El Guapo's birthday party, as he stands to open his present, his chair gets stuck to his behind and Jefe is forced to pull it off for him.
During the first strike on the village, the church and some houses are destroyed with dynamite by the bandits. However during the final fight scene, when the bandits are about to enter the apparently deserted village, the church is seen intact. However, immediately following the first attack, the villagers are seen repairing the damage.
The telegram that Lucky reads aloud is different from the "10-peso version" created by the telegraph operator. However, it was sent by Morse code and it may have been restated or misinterpreted.
There does not appear to be any thread in any of the sewing machines during the costume creating scenes toward the end of the movie.
When Dusty swings into Carmen's room at El Guapo's hideout, the very top of the rope moves all the way across the screen seeming to indicate there is no ceiling.
When the Three Amigos are escaping on the airplane with Carmen, the string holding up the plane is visible when it curves to the right.
When the Three Amigos jump onto their horses from behind, you can see three mini tramps behind the horses.
As the Three Amigos fly into the village of Santo Poco, a right banking turn, wires attached to the plane can be clearly seen flashing under studio lighting, revealing the scene to be a studio model.
After Carmen knocks out the guard and she tells Dusty to dress in his clothes she hands Dusty his gun. Dusty spins the gun in his finger but misses his holster. The bottom of the pistol grip is pointing to the sky indicating it's not in the holster, just laying on top.
After Dusty and Carmen knock out the person guarding her room, Carmen says "quick, you must dress in his clothes so you can move freely though the courtyard." Chevy Chase is a 6'4" white guy trying to hide his identity in courtyard full of Mexicans who mostly know each other. There is no way the disguise would work.
Although the film is set in 1916 as stated at the beginning, jet contrails can be seen in the sky as the boys are singing the "Amigos" theme before the opening credits.
The German mail aircraft was a red, black and white Bücker Bü 131, a 1930s basic training aircraft used during World War II. The movie takes place in 1916, but this aircraft had its first flight on April 27, 1934. The aircraft is called a Tubman 601, although there is no real aircraft with that name.
In his retelling of a story of meeting Dorothy Gish, Ned Nedlander relates how the actress calls him "Young Man". The film is set sometime around the 1st world war, the story quite a while before. Dorothy Gish did not turn 20 until 1918. Martin Short was 36 when the film was shot.
When Conchita is explaining about El Guapo and foreplay to Carmen, a copy of Warner Sallman's famous "Head of Christ" painting is on the wall. The movie takes place in 1916, but Sallman didn't paint the Head of Christ until 1940.
When the three amigos are singing by the campfire, Dusty is holding what appears to be a nylon-string guitar. When he starts to play Blue Shadows, the sound we hear is from a steel-string guitar.
In the wide-screen DVD version, in the Singing Bush scene there is crew and equipment visible on the left side.
When the three amigos are singing by the campfire, and all the animals are gathering near, you can see the shadow of a crew member on the right side of the screen.
The citizens of the village state that they do not have weapons to fight El Guapo, but in the final battle, when they all dress up like the Amigos, almost all of them have revolvers and ammunition.
El Guapo states that he will go to the village tomorrow with "50 of his men" and later Carmen says that "El Guapo with all his men" are just outside the village.
However, the next morning there are only 22 men, including El Guapo, facing the Three Amigos.
However, the next morning there are only 22 men, including El Guapo, facing the Three Amigos.
At the birthday party, El Guapo says he knows all his men as well as his own smell, yet in the previous scene, he has to ask Jefe who Paco is.
When the Amigos return to town after El Guapo's rampage, they go to Carmen's parents home to "retrieve their things." But the Amigos arrived with nothing but their Amigo costumes. Nothing else. And when they ride off into the sunset, they have nothing with them.
When Carmen goes into the church and crosses herself at the beginning of the film, she does it incorrectly. She touches he right shoulder before her left; this is the Eastern Orthodox style, but Roman Catholics touch the left shoulder first. (The young boy with her does it correctly.)
The bandits have cartridge belts with ammunition calibers for which they don't even have weapons. The cartridges appear larger than the barrels of the revolvers.