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.
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.
When Fezzik leans Westley on the castle wall, the wall moves.
When Buttercup is talking to the Man in Black on the hill, some obviously modern houses are visible in the valley behind her.
When Westley is fighting the ROUS, you can see a techie by the tree step in and out of the picture.
When Inigo, Vizzini, and Fezzik kidnap Buttercup in the woods, Inigo looks like a cardboard cut-out, but in fact can be seen to blink. He's apparently just standing very still.
When Wesley is fighting the ROUS, he punches one in the head and then you see the ROUS's hands pull the headpiece back on and straighten.
During the fight with the ROUS in the fire swamp, when the ROUS attacks Buttercup and bites her dress, you can see the silhouette of a crew member's hand holding the dress in the ROUS's teeth.
When the Man-in-Black knocks Fezzik out and is rolling him over, there's a large rock in the way. At the end of the shot, the rock has moved over to allow room for Fezzik's head.
When Inigo hears Westley's cry of anguish (when the latter is subjected to the highest level in The Machine by Humperdinck), he explains that he knows the cry was made by the Man in Black, because the latter's true love is marrying someone else later that day. But Inigo has no way to know that Buttercup is the Man in Black's "true love".
Princess Buttercup's dress doesn't have scorch marks after it is burned in the Fireswamp.
When Westley, Inigo and Fezzik storm the gate guarded by 60 men, Fezzik is set on fire. But in multiple shots where he is far away, you can tell it's a dummy.
Obvious stunt performers (one with a mustache) when Buttercup pushes Westley down the hill, and then follows him down.
When duelling, Inigo and the Man In Black sometimes have two shadows.
When the camera shows Westley screaming in pain on the Table, you can see modern silver fillings in several of his molars.
When Fezzik and the others storm the castle, camera shots taken from their perspective, i.e, looking toward the mob and castle beyond, the castle's gates are securely shut. However, the shots of Fezzik from the defender's perspective are taken through the wide open gates.
When Westley fights Inigo, Fezzik, and Vizzini and when he runs off with Buttercup, he is wearing gloves.They are prominent when his fist passes Butturcup's face, which he describes as a warning. When he is pushed down the hill his mask and bandana come off as he rolls 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.
When Inigo meets Count Rugen for the first time, it is just after Humperdinck and Buttercup's wedding, which is at night. Yet a window behind Count Rugen clearly shows it to be day.
When Inigo jumps against the door to get at Rugen, the whole wall moves. Clearly it isn't a real stone castle wall.
Westley's mustache changes throughout the film, sometimes it is uneven, sometimes it's longer on the sides than at other times.
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.
On the castle wall when Fezzik and Inigo give Westley the miracle pill, Westley's right hand is by his side in some shots and resting on his chest in others.
The dust on the Man In Black's clothes shifts constantly throughout the duel with Inigo.
In the battle of wits scene, the knife that is near the three apples moves in different shots.
When Westley is wrestling the R.O.U.S. in the fire swamp, the tube which shoots the blast of fire is clearly visible just before the R.O.U.S. is set ablaze.
When Fezzik carries Buttercup aboard the ship, he is clearly carrying a dummy.
The car at the head of the bed moves. Also the toys including Captain America and Masters of the Universe figures move.
Vizzini leaves his boat docked at the base of the rope before climbing, yet when the Man in Black arrives moments later, the boat is no longer there.
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.
During the battle of wits, the blood on Buttercup's neck disappears and reappears between shots.
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.
(at around 1h 2 mins) When Fezzik is waking up Inigo from his drunken state with dunking his head repeatedly in water, Inigo's hair keeps changing on his face between cuts.
While the Man in Black is on Fezzik's back during their fight, one of Fezzik's lines is heard while his lips aren't moving at all.
Obvious stunt rider for Fezzik in the final scene.
On the castle wall, Inigo says "I'll say" without moving his lips.
A "He-Man" figure by the grandson's headboard is standing and lying down in different shots.
When the giant and Montoya go to the edge of the Cliffs of Insanity, to look down and see if Wesley is still climbing up, the last rocks they step on flatten, indicating they are made out of dense foam.
When Fezzik is being suited up to climb the Cliffs of Insanity the front of the boat gets caught on a rock revealing that it is not made of wood.
When Westley is being tortured in the Pit of Despair, suction cups are attached to his chest. When Westley is rescued later, it seems the petechiae/bruises from these suction cups are only on his arms. However, this is not bruising from the suction cups, this is the injury from the ROUS.
When the albino is cleaning Westley's shoulder in the pit of despair, Westley's shoulder is in various states of bloodiness.
When Fezzik is climbing the Cliffs of Insanity, on the long shots it is obvious that he is carrying dummies on his body instead of Inigo, Vizzini and Buttercup.
Fezzik obviously being pulled up the cliff by a wire.
When Inigo is being nursed back to health by Fezzik his wet hair changes drastically between shots.
When Inigo is explaining why is is looking for a six-fingered man, he says how the man had his father make the sword he is holding but then returned to get it and demanded it be sold for one-tenth the price. If he came to get the sword, why didn't he take it?
In the fire swamp, after rescuing Buttercup from the lightning sand, she and Wesley are hugging. In shots where you see her face, her left arm is over his right - but when you see his face, the arms are reversed.
As Vizzini, et al, are waiting at the top of the cliff for the Man in Black to finish climbing, Vizzini wants to continue on and leave Inigo to battle the Man in Black. He becomes impatient and lifts his arms and tilts his head to his left and says "I'm waiting!". The scene cuts to a far away shot and we can see Vizzini with his arms still raised, but his head is not tilted to his right.
During the battle of wits, when they take their drinks, a distinctive pattern of stains and marks is visible on the base of each chalice revealing that they both drank from the same prop.
When Montoya and Westley fight the rope is wrapped 4 times around the rock and comes off the near side. When Humperdinck gets there the rope is around twice and comes off the back.
When the princess falls down in the lightning sandpit, leaves are in the sand left after she has gone under. In the next shot when Westley is about to follow her, the leaves are gone.
The state of the Grandson's peanut butter and jelly sandwich.
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.
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.
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 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 the grandson and grandfather argue over whether life is fair, in shots of the grandfather's face the pages of the book are flat. When the camera turns to the grandson, the top left page is sticking up and has the grandfather's thumb under it.
In the Fire Swamp, the first flame spurt ignites Buttercup's dress, despite being several feet away from her.
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.
After rescue from the lightning sand and they are hugging, in a shot over Buttercups shoulder Wesley is caressing her face - but when it cuts to a wider angle from behind him, his arm is down at his side.
When the Grandfather turns to the Grandson at the end he is wearing a jacket with stripes. Immediately before and after he is wearing a plain brown jacket.
When Vizzini cuts the rope at the top of the Cliffs of Insanity, he cuts if very close to the boulder it is tied around. Later when Inigo retrieves the rope to lower it down to Westley, the cut end of the rope is now several feet from the boulder.
After Fezzik dumps Inigo's head in water repeatedly to sober him up, Inigo's hair changes from take to take.
Multiple items shift positions in the grandsons room: the duffel bag and hardhat on the wall behind the bedroom door, a scarf and pair of roller skates on the shelf beside the TV, a T-shirt draped over the chair the grandfather sits on, and a baseball glove positioned on a bat in the corner between the bed and the window.
The belt/sash Buttercup wears changes right around the 14-minute mark. When she meets the trio in the forest, her belt appears to be gold and braided, worn empire-waist style under her breasts. It later becomes a brown beaded belt worn around her waist. It is unmistakable if you compare 7:40 vs. 38:41.
After Buttercup is taken on the boat, there is a sandbag on the starboard side that disappears just before she jumps overboard.
When Inigo is pulling Buttercup up onto the top of the cliff, as he is pulling her up, she grabs onto his right arm. The angle shifts, and she is holding his left arm.
As Inigo and the Man in Black are dueling, there are two large stones near the base of the steps that change position.
While climbing the cliffs of insanity, Buttercup's arms alternate from down at her sides (during the wide dummy shots) to wrapped around Fezzig's shoulder. It would be impossible for her to remove her arms from around his shoulder, as her hands are tied.
The vial and stopper used to contain the iocaine powder shift around on the makeshift table throughout the scene.
When Vizzini pours the wine into the first goblet, we see clearly in the wide shot that he pours only a tiny dollop - about a tablespoon. When we switch to the close up shot, the goblet is full.
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.
When Wesley was trying to try climb up the Cliffs of Insanity there was a real easy way they could have made him fall. All they had to do was throw rocks down at him. Espically since Fizzik could throw huge rocks.
Buttercup's hair and shadows on her face during the match of wits.
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.
At the end of the movie Grandpa is wearing his tie backwards with the narrow end in front and the wide end in back with the back side of the wide end showing.