Continuity mistake: In the final leg of the race, after Frank has taken the saddle off of Hidalgo, there are many shots of Frank riding Hidalgo. In one of the slightly wider shots that shows Frank, Hidalgo's back, and Hidalgo's neck, you can see the back of an English saddle under Frank's butt, even though he's supposed to be riding bareback.
Continuity mistake: After leaving the blonde woman's tent, Frank approaches Jazira who is standing with Hidalgo. As Frank approaches, Jazira's veil is high enough to obscure the lower portion of her eyes. When the shot changes to a close-up of her face, the veil is so low that the tip of her nose is exposed. Over the next few shots, this condition goes back and forth repeatedly. (01:28:40)johnrosa
Continuity mistake: If you pay close attention, you'll see that the pattern on Hidalgo's coat changes slightly several times throughout the movie, depending on what kind of shot it is. There was more than one horse portraying Hidalgo, one appeared to be an appaloosa/paint cross and one a paint. Still a mistake though.
Continuity mistake: In the scene where Frank is about to shave, he brushes cream over his lower right cheek and entire chin. When the shot switches back to him, he is reaching for a towel to wipe the cream off, but already there is much less, including none at all on the left half of his chin.
Continuity mistake: In the scene where the evil Sheik kills the cattle boy when he is cutting a deal with the woman the body falls right in front of her but when the Sheik rides away the body is gone.