Continuity mistake: When Westley and Inigo are duelling at the Cliffs of Insanity, Inigo makes a strong gesture (feet together, right arm holding sword extended downward, left arm up extended backward). When the camera angle changes, the duel picks up a couple of seconds before that stance and he does it again.
Continuity mistake: This is the scene after Westley gets tortured and he, Indigo and Fezzik are on the stone wall in front of the castle. Fezzik says, 'You moved your head, does that make you happy?' Wesley sort of turns his head and it falls in front of him towards his left. In the next shot, his head is against the wall leaning to his right.
Continuity mistake: At the end of the film, when the grandfather is leaving the boy's room, his mustache and hair aren't grey anymore, they are both brown. His mustache is much thinner too, and his hair isn't as wavy as in the rest of the film.