Revealing mistake: During the fight scene in the belly dancer's room, Bond spins the bald guy around to avoid being hit by a chair. When the bald guy gets hit in the back with the chair, you can see a large rectangle of padding under his jacket.
Revealing mistake: When Clint Eastwood and Jeff Bridges are being chased by George Kennedy and his partner, Bridges is driving off the dirt road down and embankment and you can clearly see that in the passenger seat is a faceless dummy instead of Eastwood or another stunt man.
Revealing mistake: When D'Artagnan and Rochefort are sword fighting on the frozen pond, both fall through the ice in separate places. As each one flails about in the water, it's obvious that the broken ice pieces are Styrofoam. It also appears that what is supposed to be snow covering the ice is ground up Styrofoam, too chunky to be real snow.