Visible crew/equipment: After Wesley sets the ROUS on fire, as the ROUS is writhing on the ground, you can see a crew member (fire safety?) walk into view beneath one of the roots of the trees on the right-hand side. (00:47:35)
Continuity mistake: After Montoya has hauled Wesley to the top of the cliff, he drops the rope on the ground, where it lands forming a couple of coils. You can see this as Wesley sits down and Montoya leans up against the wall. However, when they start their duel, the rope is hanging straight off the cliff, without any coils in it.
Revealing mistake: 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.
Continuity mistake: 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.
Revealing mistake: In the scene where they're tumbling down the hill, Buttercup has a beard.
Continuity mistake: 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.
Other mistake: 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.
Revealing mistake: 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.
Revealing mistake: When Fezzik is pulling everyone up the Cliffs of Insanity, look closely and you will see the cable that is pulling HIM up.
Visible crew/equipment: 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?
Continuity mistake: 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.
Continuity mistake: 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.
Continuity mistake: 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.
Continuity mistake: 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.
Visible crew/equipment: 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. (00:24:25)
Visible crew/equipment: 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_matte.shtml for an illustration. Still a mistake, but there's why - pan and scan is evil.]
Continuity mistake: When Fezzik falls to the ground after fighting with Westley, there is a very large rock right next to him. However, when Westley tries to roll over Fezzik, the large rock is too close to do so. Therefore, the next shot shows them further away from the rock so Fezzik can be rolled over.
Continuity mistake: 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.
Continuity mistake: 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.
Continuity mistake: 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.
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.