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Continuity mistake: Ahsoka is wearing a red bodysuit for most of the episode, but when she falls to the floor after Bane shocks her unconscious, her legs are miscoloured a pale pink. (00:13:21)
Character mistake: After Anakin says to use the walkers (AT-TEs) as boarding vessels, Rex calls the first one an AT-AT (a vehicle that does not even exist at this point in-universe) while ordering a battalion of clones into it. (00:04:10)
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.