Necromancing the Stone - S5-E21
Plot hole: Piper mentions that Grams banished the Necromancer "sixty years ago" at Patty's Wiccanning. Grams soon thereafter says that the Necromancer has had "almost seventy years" to regain his strength. However, Patty was born in 1950, and this episode aired on 5/4/03, at which time Patty's Wiccanning could not have been more than 53 years earlier.