Corrected entry: When Westley and Inigo are dueling, Westley disarms Inigo by knocking his sword high into the air, which Inigo subsequently catches. But a crew member caught the sword and dropped it back down to Inigo, and you can see him running away at the very top of the screen, far to the left. (00:24:21)
Corrected entry: When Vizzini accepts the battle of wits with Westley, Westley states that the poison he has is "called iocane powder. It is odorless, tasteless, dissolves instantly in liquid, and is among the more deadlier poisons known to man." How is that when Humperdinck finds the empty poison packet later on, he identifies it by sniffing it? Pretty clever for an odorless poison . . .
Corrected entry: Look closely at Buttercup's wedding dress at the actual "wedding". It's ice blue with a white petticoat or whatever under it. It stays the same until where she jumps out the window in the end and lands neatly in Fezzik's arms. Watch her falling - her dress changes to purest white to contrast the night sky better.
Corrected entry: 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.
Corrected entry: When Grandpa is reading to the boy, first the boy's sandwich is whole, then there are some bites taken, then it's whole again.
Corrected entry: In the final scene with the grandpa and the boy the grandpa's hair gray (as it was in the rest of the movie) but during a close up when he at the door about to leave his hair is dark brown. It changes back to gray right before he leaves.
Corrected entry: After Buttercup realizes that Humperdinck never sent the messengers and is calling him a coward, he grabs her and shuts her in her room, intending to lock her in since he grabs the key out of the lock. However, he immediately pulls the key out after shutting the door without turning it first- so the door is actually still unlocked. Since Humperdinck was intending to lock her in, it's unlikely that the character would have 'accidentally' forgot to lock the door, more likely the actor was just moving too fast.