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: 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)