In the sword fight between Man in Black and Inigo, when MIB throws his sword down and then proceeds into his flip from the "horizontal bar" onto the ground level, he lands a good five feet from the thrown sword. In the next cut, he reaches a foot away to grab the sword's handle.
When Westley fights Inigo, Fezzik, and Vizzini and when he runs off with Buttercup, he is clearly wearing gloves. When he is pushed down the hill we see his mask and bandana come off and when we see his hands again his gloves have disappeared and he is without them for the rest of the film.
After Buttercup pushes Westley down the hill, she throws herself down after him. Towards the bottom, her sash falls off and she's left laying on the ground. However, when Westley goes to speak to her, she is wearing the sash around her waist once more.
In the opening scene, the camera is behind Grandpa, looking over his shoulder, when he pulls out his glasses out, the book is open. When the camera is in front of him as he puts his glasses on, the book is closed.
When Buttercup pushes Westley down the hill and she falls down after him, when they both come to a stop Buttercup ends up several feet farther down the hill then Westley. The movie then cuts to a scene of Humperdinck, then back to Buttercup and Westley. However, they are now shown to be right next to each other.
Clear landing pad in the first fight scene when they jump off the rock. The landing is elevated and one can see the rubber mat ripple as they land. The pad can also be seen when Inigo and Fezzik walk to the edge to look down at Westley climbing up.
Prince Humperdinck correctly identifies the iocane powder after smelling it, even though the Man in Black had earlier described iocane as "odorless". However, the fact that it is odorless is likely why he "bets his life on it" to be iocane powder.
Vizzini mentions Australia's convict colonies during the movie. This may seem anachronistic since the movie has a medieval setting, but this story is fantasy not history. In fact, its being told as a fairy tale from a grandfather to his grandson. In the novel, this was just one of many (intentionally contradictory) clues about when the story takes place.
In the king's palace, Inigo battles four of Count Rugen's henchmen. He stabs all four of them, yet there is not a drop of blood on his sword. Later, while battling Count Rugen, his sword suddenly has a lot of blood on it, but this is before he has stabbed Rugen. Meanwhile, Rugen's sword has blood on it from stabbing Inigo, until it all disappears in one shot, and he's fighting with a clean sword.
In the final scene, in the close up of the grandfather saying, "As you wish," the wall color is different from the color of the wall in the wide shot. This is because the close-up was a pickup shot done well after the original scene was shot.
Inigo confronts Count Rugen and four attackers while Fezzik holds the man in black. Inigo kills the four attackers and chases Rugen. Whan Fezzik returns to find the man in black after breaking the door down for Inigo, the bodies of the four attackers are no longer in the same position that they were when Inigo killed them.
Count Rugen is kicked after being stabbed by Inigo. He falls back against one of the dining hall tables, and his sword slides out of his hand. In the next shot however, just before he hits the ground, he is holding the sword again.
In the Pit of Despair, Humperdinck stands beside Westley as he lies on the table before The Machine. The dual-ended hose on the suction cups strapped to Westley's chest are facing forwards, away from his shoulder. But in the next shot, when Humperdinck talks to Westley, the hoses are facing backwards, with the hoses facing his shoulder.
Count Rugen hits Westley on the left side of his head, towards the middle, to knock him out before sending him to the Pit of Despair. In the Pit, the bump on his head is on the right side of his forehead, but this might be from his fall.