What was especially bizarre and unfair about the accusation of Rebecca Nurse?

Rebecca was accused to have "appeared to the said afflicted persons [the girls], in the form of a black man, with a spindle."

In Puritan theology, Satan frequently appeared as a black man. As proof of witchcraft, accusing others of walking with a black man was almost too simple. If Satan appeared to a witch as a black man, his appearance to the girls would surely seem to be real. Thus an effective tool for false accusation was in the girls' hands. Rebecca did not stand a chance.