Old Friends Not Forgotten - S7-E9
Continuity mistake: When Cody is telling Obi-Wan about the tactical droid remaining out of sight, the armour accessory he wears on his back is missing. (00:02:05)
Old Friends Not Forgotten - S7-E9
Continuity mistake: When Ahsoka and Rex watch from their gunship as another one is shot down on the approach to Sundari, his kama is missing. (00:18:24)
Old Friends Not Forgotten - S7-E9
Continuity mistake: When Bo-Katan storms the throne room, the pistol she's holding in her right hand is kicked out of her hand by one of the supercommandos. However, when she goes to fight Almec, there's a pistol in her right holster despite her not having had time to retrieve the weapon. (00:23:30)
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.