Deliberate mistake: Pre Vizsla's appearance in the hologram doesn't match up with his appearance in the previous and succeeding episodes, as he's wearing clone armour covered with a cloak that has a different symbol on it. This is because when this episode was being produced, Vizsla's final character model wasn't complete yet. (00:16:25)
Deliberate mistake: At the beginning, Asajj Ventress is wearing an outfit with a long skirt. When she is rescued from her damaged starfighter by the scavengers, the skirt has vanished. This was done because the animators found the skirt too difficult to animate. This is also why she receives a brand new outfit that never had a long skirt later in the episode. (00:08:20)
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.