As Brownlee inspects a row of slaves in the film's first scene, he smokes an obviously-lit cigar, but when he clenches the cigar in his mouth and leans in to inspect a slave's teeth, the cigar changes to one that has never been lit; maybe as a precaution against burning the other actor's face.
About 18 minutes in, when Dite tells Hammond that she's "knocked up," her leg changes position between shots.
During Mede's first fight in the city, he and his opponent are wrestling on the grass. When they roll over on the floor, the grass moves like a slipping carpet would, revealing it to be a sheet of artificial green, probably lying on the floor of a sound stage.
In the final scene, it is evident that sublimating dry ice is being used to simulate steam rising from the "boiling" cauldron.