Zelda reads a notification advising Sabrina that Harvey exceeded a "Spell Quota". She clarifies that he is now immune to magic and will be aware of any future spells cast around him. This is how Harvey discovers that Sabrina is a witch. However, Harvey later falls victim to her "pinch of salt" spell (which causes victims to become distracted by talking about salt) without realizing it.
In season 5, Roxie tells her housemates that she dated Isaac Hanson before he became famous, and they have remained friends. This is proven to be true with Isaac meeting up with Roxie before performing with his band Hanson. Roxie then forgets all about her past involvement with Isaac. She tells Sabrina that she once ran away from home to see Hanson but never got to meet them. Sabrina and several others were there when Roxie last met the band, and talked directly with Isaac.