In this Spanish TV movie (also known as "Películas para no dormir: Regreso a Moira"), a beautiful but lonely woman named Moira (Natalia Millán) is suspected of being a witch because she has a cat. There's a lot more to the story than that, but the cat is the only physical evidence of the alleged witchcraft.
Watching young Tomás (Juan José Ballesta) from the chair beside its mistress' bed isn't the only creepy thing this white cat does. Apparently, it can also change into a longer-haired version of itself as a continuity error.
Also look out for when the much older Tomás (Jordi Dauder) has a very blurred white cat with a ginger stripe along its back pass behind him as he pays a final visit to his friend's restaurant. Maybe it's the slightly changed ghost of Moira's cat who also, unfortunately, came to a tragic end over 40 years ago.