When Marguerite and Armand look at the chateau, there is a large tree behind Armand. In the next shot from the opposite angle, there is no such tree, it the most obvious indication that the two angles are of different sets.
Prudence raises her left hand to her cigar, but removes it from her mouth with the right hand.
The name CAMILLE--a Latin name--has nothing to do with the camellia of the Lady of the Camellias, which was so named by Linnaeus in honor of the Czech botanist Jiri Josef Kamel.
When Marguerite and Armand go walking through the field in the countryside, he asks her, "Tired?" When she responds "only mildly tired," her lips do not move.