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.
Revealing mistake: In the scene where they're tumbling down the hill, Buttercup has a beard.
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.
Revealing mistake: 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.
wizard_of_gore
Revealing mistake: In the scene before the climb of the Cliffs of Insanity, 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.
Revealing mistake: 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.
Revealing mistake: When 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 than it is in the medium shots.
Revealing mistake: 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.
Revealing mistake: In the scene where they ride out of the castle on horses, it's obviously a smaller double for Fezzik.
Revealing mistake: In the shot where Fezzik is putting on the harness in preparation for climbing the Cliffs of Insanity, you see the boat start to drift away. As it does, the figurehead catches on the rocks and wobbles, as if made of rubber. (00:13:25)
Revealing mistake: When Buttercup's dress catches fire in the fire swamp, Westley quickly puts it out. Light is shining through the skirt, and you can see a shadow of a hand reach out and hold the skirt so he can do so.
Answer: Not meaning to shoot down your idea, but since he is wearing gloves, even if the gloves only had five fingers, it would still not be a mistake as he could easily tuck his two smallest fingers into one of the glove's fingers. I would guess that this is why he wears gloves throughout the film, as the director would surely not have gone to the trouble of making 6-fingered gloves, and the actor does not really have 6 fingers.
Jazetopher