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)
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?
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.]
Visible crew/equipment: 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.
Visible crew/equipment: 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.
Visible crew/equipment: During the sword fight, every time they do a large jump you can see a crash mat under the sand. (00:22:17 - 00:24:56)
Answer: He is just singing/babbling happily. According to the captions, he sings, "Hi diddle lick do day dee shie!" He then says, "That is a noble cause".
BGraz