In the episode "Just harried" (Season 3) When Piper gets married Penny Halliwell (Grams) says that the Halliwell women never change their last names just like she didnt. But in the episode "That 60's Episode" (Season 6) she said she had married her first husband with the last name Halliwell but her last name was Johnson. That dosent make scense since shes a witch and she claimed that her maiden name was Halliwell.