With the exception of a sawed-off shotgun used by one of Jackson's men, the firearms shown throughout the film are historically inaccurate for a film set in the years immediately following the American Civil War. These weapons are the Colt Single Action Army (first made in 1873) and Colt New Service (1898) revolvers, Django's machine gun (a fictional model with a barrel based on the 1866-71 Montigny mitrailleuse, but a firing mechanism and belt-fed magazine inspired by the 1895 Maxim gun), and the Winchester 1892, 1894 and 1906 lever-action rifles.
When Django discusses his machine gun and plans to take Major Jackson's gold, a considerable amount of background chatter is heard. However, few, if any, of the bar patrons are actually engaged in conversation--they are instead watching Django and Hugo's own conversation.
When Django pulls out his coffin to show Hugo and his men in the Italian version, Hugo's jaw can be seen moving, but no words come out. This is not present in the English dub, where Hugo is heard saying, "What the hell is it?"
After Django kills four of Jackon's men in the bar and twirls his revolver's trigger guard on his finger, he (uncharacteristically for a highly skilled gunfighter) fumbles while trying to pull back the hammer when aiming the revolver at Jackson himself.
After Django smacks Ricardo with the butt of his own rifle and throws it back to him, Ricardo angrily readies the rifle with the intention of shooting Django. Although a metallic clicking sound is heard, he is actually shown to be miming the action (presumably by mistake)--his hand is not within the lever.
In the town shootout where Django assaults Jackson's Klansmen with his machine gun, many of the Klan members are shown being killed in multiple shots at different times (for example, a Klansman in a light gray buttoned-up jacket near the front of the gang is killed at least four times).
Promotional materials for Blue Underground's releases of the film proclaim that the Italian version uses Franco Nero's voice for the character of Django. This is incorrect--Nero was, in fact, dubbed by actor/voice actor Nando Gazzolo.
According to the subtitles of Blue Underground's DVD release, General Hugo describes Maria as being "half Incan and half Mexican", despite his actual dialogue indicating that she is "half Yankee". This was corrected for the Blu-ray's subtitles.
The bartender Nathaniel accompanies the piano-player on the fiddle, but his violin has no bridge; the strings run off the end of the fingerboard and directly to the tailpiece. The most any player could get out of such an arrangement would be a couple of high-pitched squeaks.
The belt feeding the cartridges into Django's machine gun is far too short for the number of rounds being fired in any of the scenes it is used in, and the belt isn't being fed through the firing mechanism.
The goof items below may give away important plot points.
When Django uses his revolver to gun down Major Jackson and his remaining men, he kills all six (according to the sound effects heard) with seven shots. A revolver can only fire six bullets before it needs to be reloaded.
When one of the Mexican officers is shown firing his revolver at Hugo's revolutionaries during the ambush led by Major Jackson, the gunfire sound effect heard has a tinny, high-pitched quality. However, when the same officer is later shown firing the same pistol, the sound effect is an entirely different, lower-toned gunshot.
When Major Jackson says " . . . the son" when shooting the sides of Mercedes Zaro's cross during the final duel in the Italian version, his lips don't move. This does not occur in the Englisn dub, where he has no dialogue at this point.
When Jackson and his men go to Nathaniel's bar whilst searching for Django, Jackson has five men with him. However, when Jackson confronts Nathaniel in the bar, that number increases to six, before dropping to five when they go to the cemetery.
After Maria is shot by Hugo's men, the position of her body changes from being horizontal, then vertical, then horizontal, in relation to the positioning of the bridge. Although she is revealed to have been wounded, rather than killed, it would be improbable for her to move about like this.
When Django first fires his revolver at Jackson and his men in the cemetery, the barrel is within the metalwork on the side of Mercedes Zaro's cross. The barrel later switches to being under the metalwork, before returning to being within.
Major Jackson has been leading a Ku Klux Klan-style gang that targets Mexicans because of racial reasons and is "killing arbitrarily all those who aren't white skinned". Later, Jackson leads Mexican soldiers into battle against Hugo's gang. This is to show the hypocritical nature of tyrants and megalomaniacs who cast aside treasured creeds and core ideologies for profit and convenience. This logic also explains why Jackson and his men have no objection to Nathaniel employing a full-blooded Mexican prostitute.