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 fence around what looks like a tennis court in front of the mansion is chain link fencing.
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).
Several times when characters open their mouths wide (either to bite or to scream), modern fillings are seen in their teeth.
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.