In the movie's first scene, as Brownlee inspects a row of slaves he smokes a cigar which is obviously lit (smoke, ash, etc). 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 a precaution against burning the other actor's face?)
During Mede's first fight in the city, he and his opponent are wrestling on the grass. When they roll over the floor, the grass is moving like a slipping carpet would, revealing it to be a sheet of artificial green, probably lying on the floor of a sound stage.