Continuity mistake: R2's squad of battle droids has three members. Yet in the climactic fighting of the previous episode, one of the droids was shown getting destroyed.
Continuity mistake: When Ahsoka is boarding the rescue gunship, her normally green lightsaber has a blue blade. (00:18:00)
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.