I've Got You Under My Skin - S1-E2
Corrected entry: Prue smugly chastises Phoebe for trying to use magic for personal gain when she plays the lottery numbers from her vision. Not addressed in the episode is the fact that Prue used her telekinesis to prevent the elevator from stopping at everyone else's floors when she was on her way up to her interview at Buckland's.
Correction: Prue is being hypocritical, but that hardly ranks as a mistake. The issue of her using her powers for personal gain is never addressed because none of the other sisters see it.
Shay