When Ingrid Pitt is lying in the coffin at the end of the film, she is wearing a nightgown with smooth fabric on the chest and stomach. As she gets Staked, the fabric is pleated.
If this movie takes place in 1790, then how could there be a motor? The automobile was not invented until the mid-late 1880's.
The story is set in the late 1700s, but at the General's ball the orchestra plays a couple of pieces by Johann Strauss Sr. (1804 - 1849) and Léo Delibes (1836-1891).
The fence around what looks like a tennis court in front of the mansion is chain link fencing.
Bubble bath scenes are great, but the bubble bath is a 20th century invention, and Carmilla is shown taking a bubble bath in the 1700s.
At 1:13 the character Renton appears visible in a mirror. The vampire Camilla approaches Renton and also appears in the mirror reflection. In the 1700s a vampire would cast no mirror reflection.