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.
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 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.
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 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.
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.
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 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.
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.
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.