In the scene where they're tumbling down the hill, Buttercup has a beard. In the scene where they're tumbling down the hill, Buttercup has a beard.
When Westley and Inigo are duelling, Westley flings his sword so that it sticks into the ground and then athletically flips from a bar to where the sword is and nonchalantly reaches down and back and takes it out of the ground. But if you watch, he lands too far away from the sword to do this.
When Westley and Buttercup are in the fire swamp and Buttercup is burned for the first time look carefully at the fire spurt, it's several feet away from Buttercup and is clearly too far away to have burned her.
When Vizzini is cutting the rope at the top of the cliffs of insanity he cuts it really close to the rock the rope is wound around. In the following shots there is a lot of length of rope between the rock and where it was cut.
When Fezzik falls to the ground after fighting with Westley, there is a very large rock right next to him. It is gone in the next shot.
When Westley and Buttercup are walking through the Fire Swamp Westley's hair continually gets better or changes. One scene it is slicked back and another it is in his face or over to one side, etc.
There is a scene where Westley is matching wits with Vizzini. In every shot of Vizzini and Buttercup, her hair changes. In one shot, half of it is over her shoulder; in the next shot it is all down her back. This happens all through the scene.
When Inigo is lying aginst the wall after Count Rugen has stabbed him in both shoulders, back shots of him show his head is tilted to the left and bobbing while in front shots, he is holding it straight.
Fezzik tries to crush Westley on a boulder. We see him doing the crushing spread-eagled, but when the angle changes to a close-up, he is holding Westley's wrists tightly under his chin.
Watch the red car behind Fred Savage in the bedroom. In close shots, the car is pointing towards the camera. In wide shots it's pointing sideways.
When Buttercup jumps down the window, she is falling in a vertical position but in the next shot, just before she falls in Fezzik's arms, she is suddenly in a horizontal position.
When Inigo and Westley are duelling, they are making athletic jumps. One does it after the other and when they land you can see the mat wrinkle under the "dirt" on the ground.
As Fezzik, Inigo, and Westley are developing their plan to storm the castle, Inigo says Humperdinck and Buttercup are getting married in a 'little less than half an hour'. It is bright and sunny outside. Next time we see them, before their master plan is carried out, it is dark, like it's the middle of the night.
There is a scene where the Prince and the Count are talking at the entrance of The Pit of Despair and the sound mike can be seen bobbing around on top. [This is only visible in the original 1980s VHS pan-and-scan version of the film, and is due to the open-matte process that was used to produce the VHS release. See http://www.widescreen.org/widescreen_matted.shtml for an illustration. Still a mistake, but there's why - pan and scan is evil.]
When Fezzik is pulling everyone up the Cliffs of Insanity, look closely and you will see the cable that is pulling HIM up.
When Inigo and Count Rugen are duelling in the banquet room, watch the Count's sword: after he stabs Inigo near each shoulder and draws out the sword it is clean. A moment later, as Inigo falls forward a bit and then rights himself, blood is seen on the Count's sword. If you watch both their swords during this entire scene you'll see the disappearing/reappearing blood.
After Fezzik has finished dunking Inigo's head in the bucket of water, Inigo has hair stuck across his forehead. While talking to Fezzik Inigo's hair suddenly is all brushed away from his face. Inigo's hair changes back and forth throughout the rest of the scene.
After the grandfather has finished reading the book, he stands up to put on his hat and coat. The shot goes to behind him, and we see that he has already put his left arm through the sleeve. Then the shot changes again and we see him from the front as he is sliding his left arm into the sleeve.
In the scene where Inigo kills all of Count Rugen's guards, notice the position of the dead guard in front of Fezzik and Westley. He changes position once after the shot of Count Rugen running away, and again when Fezzik puts Westley on the suit of armour.
During the scenes when Buttercup wears a red dress (from horseback ride to Fire Swamp), watch her legs carefully. She is wearing red stockings which periodically disappear and reappear.
When the ROUS is biting Buttercup's dress (ground level shot), you can see the silhouette of a hand inside, or maybe behind, the dress. Maybe they had to push it up to the mouth because it couldn't grab it?
In the scene where Fezzik, Inigo, and Westley are on the wall after Westley is tortured, Westley's hand changes position several times throughout the scene. This is odd considering he can barely move his thumb. But it changes between at least three positions several times.
When Fezzik is about to climb the rope going up the Cliffs of Insanity with Inigo, Vizzini and Buttercup all harnessed to him, Buttercup is the most visibly seen (besides Fezzik) in the camera. When Fezzik starts to ascend the rope, it is clearly obvious that Buttercup is a stuffed dummy because her head is flopped completely back.
At the end of the movie, when Fezzik is catching Buttercup jumping out the window, just as she lands in his arms, you can see the platform she jumped off at the top of the screen.
When Westley and Inigo are duelling at the The Cliffs of Insanity - and as Inigo catches his sword that had been knocked into the air - one can see a portion of a boom mike bobbing down on the left side of the screen. Pan and scan version only.
In the scene before the climb of the Cliffs of Despair after they get off Vizzini's ship the end of the back of the ship suddenly jiggles back and forth like it's made of rubber.
In the scene where Fezzik is using the holocaust cloak, the long shots of him are a dummy due to the bobbing around (and the fact that they cover up the face). It's only Fezzik in the close-ups, where the hood is pulled back making his face more visible that it is in the medium shots.
When Buttercup finds Westley in the honeymoon suite and jumps on him on the bed, her stockings and shoes are black. When she jumps from the window into Fezzik's arms, her stockings and shoes are white. I doubt she changed in front of Westley, Inigo, and Prince Humperdinck.
During the first scene when the grandfather is reading to the grandson, he opens the book. When he gets his glasses the book is shut and he has to open it again.
When Humperdink and the six-fingered man are on horseback near the coast, discussing strategy, you can tell that the ocean is a matte painting, as the waves and whitecaps never move.
When the Albino is cleaning Wesley's wound in the Pit of Despair, watch the rag: it goes from white to deep red and back to white between shots.
When Westley and Inigo are fencing, Westley's black outfit gets dusty, and the dusty parts shift throughout the duel.
In the duelling scene between Westley and Inigo, right after Westley has done his swinging on the bar and they go into the next shot, you can see Inigo jump up on a higher platform of rock, and then immediately after him, you can see Westley BOUNCE off the concealed trampoline onto the platform of rock.
In the duel between Inigo and Count Rugen, Inigo stabs the count in the right arm. But you can tell the sword is shorter, and it obviously wasn't stuck in the count's arm when he pulls it away.
The iocaine-poisoned wine cups change position on the table between shots.
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.
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.
At the beginning when Buttercup tells Westley to fill her buckets with water, her hair goes from being all over one shoulder in her closeup, to hanging down her back as she walks away. When she turns around, it's all over her shoulder again.
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.
When Buttercup's kidnappers are arguing over whether or not they should kill her the weather goes from downcast to sunny and back again whithin a few seconds.
When Westley and Inigo are dueling atop the Cliffs of Insanity, the shadows cast change direction from shot to shot.
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.
In the scene where they ride out of the castle on horses, it's obviously a smaller double for Fezzik.
In the scene where Fezzik is putting Inigo's head in a bucket of water, every time he dumps Inigo's head in the water, you hear his deep breath. Not when his head is pulled out of the water.
When Fezzik puts the "almost dead" Westley down on the castle wall where they look out to the gate door, the wall moves. In fact, inside the castle when Fezzik breaks down the door for Inigo, the wall moves then too.
After Westley is injured in the Fire Swamp, the wound on his shoulder changes colour and position for the rest of the movie.