Continuity mistake: During the extended fight scene, Will Danaher and Sean Thornton go crashing into a hay stack. As they emerge, they announce to the crowd that they are going to fight by the Marquis de Queensbury Rules. As they make the announcement, Sean Thornton's back is covered in hay. As he turns, the camera cuts to follow him, and there is instantly no hay on his back.
Continuity mistake: When Barry Fitzgerald meets with Maureen O'Hara in the house as the shocklehorn, when they go in the house it's a clear day. As they talk in the house, every time the camera turns to O'Hara the window behind her shows a rainy day, but every other view out every other window shows a clear day.