Revealing mistakes: When Isabelle (Thelma Ritter) and Guido return to the house after Roslyn and Langland have been alone there for a few days, Langland calls Isabelle "Thelma".
Revealing mistakes: The wild horses are captured by throwing a loop around their neck. Yet, when we see one of the horses on the ground, the rope is looped around the neck AND muzzle, to fashion a makeshift halter, revealing that the horse is halter broke.
Errors made by characters (possibly deliberate errors by the filmmakers): When Gable, Monroe, and Clift look at the horses in the distance with the binoculars, all three of them hold the binoculars upside down (but they still work, of course).