Beam Me Up, Scotty!: It's "You see, my mule don't like people laughing," not "My mule doesn't like to be laughed at."
Executive Meddling: A ten-minute prologue (hastily filmed by the studio and featuring a body double for Clint Eastwood whose back is facing the camera) was attached to the film when it was first aired on network TV. Why? The guy in charge of ABC HATED the film's amoral plot and refused to air it unless the studio "fixed" what he saw was the impure motive of Joe. Hence the prologue, which rewrites the entire plot via a single ten minute scene where it is "revealed" that Joe was a former convict who was given his freedom in exchange for doing the bidding of the warden of his jail, in terms of driving out the gangs from the town in which the film takes place. Thankfully, the scene only was used for one airing and was promptly destroyed by the studio after the fact... although it has somehow made it onto the DVD as a special feature.
Fake Nationality: Everybody who isn't Clint Eastwood, most notably Italian Gian Maria Volonte as Mexican Ramon.
What Could Have Been: Just look at the list of actors supposedly approached to be Joe: Henry Fonda, James Coburn, Charles Bronson, and the lesser-known Richard Harrison (who reportedly recommended Clint Eastwood to Leone).