Revealing mistake: In the scene where they ride out of the castle on horses, it's obviously a smaller double for Fezzik.
Continuity mistake: In the scene where Westley is trying to climb the Cliffs of Insanity, the shot of him from the top shows him holding on to a flat rock surface. The shot from his perspective shows him holding on to several bulbous rocks.
Continuity mistake: When Westley and Inigo are dueling atop the Cliffs of Insanity, the shadows cast change direction from shot to shot.
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.
Other mistake: When Westley and Inigo are fighting, they are speaking very quickly to each other. They're talking about different fencing styles, and these styles actually exist. Just the same, their fencing has nothing to do with the styles.
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: When Westley and Inigo are fencing, Westley's black outfit gets dusty, and the dusty parts shift throughout the duel.
Continuity mistake: When the grandpa is still narrating, and going on about how Buttercup realizes Westley loved her, her part in her hair goes from side to side.
Continuity mistake: After Westley is injured in the Fire Swamp, the wound on his shoulder changes colour and position for the rest of the movie.
Visible crew/equipment: During the sword fight, every time they do a large jump you can see a crash mat under the sand. (00:22:17 - 00:24:56)
Continuity mistake: When Humperdink's men are clearing out the forest, Inigo is sitting against the wall of one of the houses, drunk. The door of the hut is wide open. When he lunges at the guard who tells him to leave, the door is closed.
Revealing mistake: When Buttercup's dress catches fire in the fire swamp, Westley quickly puts it out. Light is shining through the skirt, and you can see a shadow of a hand reach out and hold the skirt so he can do so.
Continuity mistake: During the poison drinking scene there's a twig on the table that keeps changing positions in every shot.
Continuity mistake: Before the wedding, when Westley moves his head for the first time it's tilted to the right. In the close-up it's turned to the left. Then the angle changes and it's turned back to the right.
Continuity mistake: The amount of leaves around the quicksand trap increases/ decreases between shots.
Other mistake: When a disguised Westley see Vezzini holding a blindfolded Buttercup hostage, Vezzini is sitting down with a blanket, some food, two goblets and a bottle of wine. There's no way that Vezzini should have these as he wasn't carrying anything on him when he got out of the boat and Fezzik took everyone up the Cliffs of Insanity.
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.
Answer: When Inigo was a child, Count Rugen came to Inigos father and requested a sword be made for him. When the sword was finished, Count Rugen refused to pay the price he originally offered for the sword. Inigos father refused to hand the sword over so Count Rugen killed Inigos father. Outraged, Inigo took the sword his father made and tried to kill Count Rugen. He has kept the sword ever since so he could use it to kill Count Rugen.