I may have imagined this as I can not find any reference to it online. I have not gone back and watched the episodes involved, but maybe I should.
This is from season 4, I think. The year they were in college. Willow met Tara in some witch club and hits it off. At first it appeared to me that there was something sinister about Tara. She was shown sabotaging Willow’s spells (to find Buffy in one episode) and seemed to be trying to cast an enchantment on Willow (did she steal some hair?).
Then Willow becomes gay and Tara becomes her regular love interest and a stabilizing force in her life. But what was she doing originally? Was it something Joss Whedon abandoned, or was it an intentional red herring?
This is revisited in the next season in an episode called “Family”
Tara sabotaged a spell intended to locate demons, because she had been raised to believe that she was a demon - this was a lie told by the men in her family to control the women