When the Source tells the Darklighter that he wants the Charmed Ones, the Darklighter replies that his arrows don't kill witches. While the poison on the arrows is only fatal to Whitelighters, they are still arrows and quite lethal to any mortal, including witches.
Phoebe describes the Source as having tribal markings similar to Balthazar, however, the Source's small and sparse markings are nothing like the bold, all-over red and black markings of Balthazar.
When the Source opens the Hollow in front of the Hollow's evil guardian the wires of the battery and light pack are clearly visible in front of the inside of the lid.
As Piper runs over to the table to get the athame, the demon infected with The Hollow tries to kill her with her exploding power. It is clearly a stunt woman running behind the sewing machine that gets blown up.