When Bart first arrives in Rock Ridge, as he walks up the stairs, a building sign in the background says "Realtor". The Realtor organization was not founded until 1909, and the term Realtor was not used until 1916, yet the movie takes place in 1874.
Hedley Lamarr has several rubber toys in his bathroom--a rubber froggie and several ducks. Although rubber toys were believed to have existed as early as the late 1800s (coinciding with the start of rubber manufacturing), the earliest models were made of hardened rubber and did not squeak like they do in the film (set in 1874). The squeakable rubber toys seen in the film were not invented until the 1940s.
After Sheriff Bart has accepted the duties of Sheriff, Reverend Johnson holds up a Bible and tells everyone pointing a gun at Sheriff Bart to "Not let anger rule the day." Then out of nowhere the Bible gets a hole shot through it. You can clearly see that no one gathered around the platform has a smoking gun. In 1874 they would have still been using black powder. The smoke would have to clear before they could if they hit anything.
Mongo is chained to the jail cell, and when Sheriff Bart throws a bucket of water to wake him up, Mongo starts to yawn and innocently breaks free of the chains. The moment immediately before and after the chains break apart, one can clearly detect where the reel has been spliced (probably so the crew could manage the effect of the breakaway chains)
When Sheriff Bart is making his ride to Rock Ridge and he encounters Count Basie, Basie's band are not playing the music heard on the audio track. This is easily recognized if you watch the band's drummer, who isn't even playing his drum kit, let alone the actual drum patterns heard in the music.
Toward the end of the film, when the fight spills onto the other set, at one point, it spills into the street. Participants in the scene are seen running out of the studio and around the street corner. As the shot pulls back to follow Hedly looking for a cab, the runners can be seen in the background, slowing to a walk, apparently thinking they are no longer visible. Once they realize they are still in the shot, they begin to run again.
When Bart and Charlie exit the chain gang scene on the handcar to check out the quicksand up ahead, they are singing "Camptown Ladies" in Bb. The next shot shows them continuing to pump away on the handcar but the key has sharpened to C.
When Bart arrives in Rock Ridge and the "Welcome Sheriff" banner retracts, it flips once, leaving some of the writing still visible. When Bart walks over to unfurl it again, it is neatly closed with no writing visible.
When Hedley re-emerges from the bathroom, we see that he has gotten hit in the face with a pie while in there and there is only a small portion of whipped cream on his face. However, when we see him hailing down a taxi in the next scene, there is a full, even layer of whipped cream on his face.
Right before, and during, the singing of "Camptown Ladies", the cowboy behind Lyle and to his left (our right) is wearing a white jacket that goes from being buttoned to un-buttoned to buttoned several times.
When Hedley Lamarr takes a piece of candy out of the jar on his desk, it is white with several thin red stripes. After he goes to the window and almost chokes on the candy, he spits out a piece that is white with a single thick red stripe.
Right before the thugs pledge their allegiance to Hedley they show various people when they show the German soldiers they have their right hands raised then when Hedley says raise your hands they raise their left hands just to be told to raise their right hands again.
During the campfire scene, when Taggart approaches Mongo, Mongo is holding a piece of bread in his left hand, but drops it Because Taggart startled him. Taggart then proceeds to beat Mongo over the head. The camera cuts to a close-up, and Mongo is seen holding the bread again. Then the camera cuts to the long shot, and Mongo's left hand is empty again.
During Lili Von Shtupp's musical performance, the soldier that provides a chair for her to rest in, throws his rifle down on the ground when explaining how "tired" she is. The rifle hits the stage floor and slides, bursting the stage light in front of him. The sound of the light exploding can be heard through the singing and music.
The movie lists Don Megowan as the gum chewer in Hedley Lamarr's line of villains. Actually, the gum chewer is _John Alderson (I)_ and Megowan was the big guy who went on stage to accost Lili Von Shtupp and got a knee in the groin. Both Alderson and Megowan were fixtures in '50s and '60s westerns.
During the scene where the gunmen are introduced to the Waco Kid, they pull their pistols out to shoot him and the Kid draws and shoots the guns out of each of their hands. The pistols are all single action revolvers which have to be cocked before they can fire and none of them are cocked to shoot at the count of three.
When the camera is panning past the bikers who are waiting to apply for the army to take down Rock Ridge, the one on the left moves the handlebars a little too high and you can see that they aren't actually connected to anything. This is not a revealing mistake, but a deliberate sight gag which showed the two "biker cowboys" who have chopper handlebars connected to not to a motorcycle, but to their holster belts, which was obviously shown on purpose as part of the gag, and which was just one of many sight gags seen while the camera pans down the line.
During the Chess game, the Waco Kid's right hand is steady as a rock, but he says he shoots with his fluttering left hand. But later in the movie when he detonates the dynamite, he shoots with his right hand. His attitude during the discussion during the Chess game suggests he's telling a tall tale.
Toward the end of the movie when we see the marquee at Grauman's Chinese Theater, the title "Blazing Saddles" jumps on the marquee, showing it was superimposed on the film and not actually on the marquee.
After Bart and Charlie have "sunken" into the quicksand and off the bottom of the screen, the light on the backdrop can be seen to fluctuate, indicating a large reflector that was not properly secured.