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.
Necromancing the Stone - S5-E21
Visible crew/equipment: When Piper hits the mirror after being thrown across the room by the energy bolt, look closely at the roof in the other room: a black rectangular hole with a stage light poking out of it is visible.
Necromancing the Stone - S5-E21
Visible crew/equipment: After Phoebe tells Jason that she'll go with him to Hong Kong in the morning, when it cuts to Piper and Paige looking out of the attic window, the reflection of a crew member is visible in the window behind the candles. (00:33:40)
Answer: That was a mistake in the series.
Also, the episode is called 'Babies First Demon' and this alone tells us that it's Wyatt's first time in the field (so to speak) and as the sisters say, they've done a good job of clearing up their own magical messes so far. Nobody really knows how advanced Wyatt's powers are, or will become. That's why the cleaners appear, to nip things in the bud. The Cleaners didn't want to chance that the sisters could clean Wyatt's magic up, just incase they couldn't. Therefore intervening themselves, just in case.