In the scene where Hedley's men ride through the fake town, there is a quick cut to Lilly Von Schtupp singing a drinking song with the German soldiers. They are singing the same song that Zero Mostel and Gene Wilder were singing with Kenneth Mars in The Producers.
Madeline Kahn started working on the movie the day after being fired from the role of Agnes Gooch in Mame (1974). Lucille Ball believed that Kahn got herself fired (by deliberately acting poorly) so that she could take the role of Lili Von Shtupp in this film, but still get paid for the Gooch role in "Mame", which by her contract wouldn't have happened if she'd merely quit.
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.
When the cowboys ride into the fake town, the buildings are simply facades. After Jim fires the gun and sets off the dynamite. The townspeople run in to "finish them off", the buildings are real.
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.
Lyle: Come on, boys! The way you're lollygaggin' around here with them picks and them shovels, you'd think it was a hundert an' twenty degree. Can't be more than a hundert an' fourteen.