Continuity mistake: Chopper's eyes are different colours, with his right eye a whitish hue. However, in the close-up of him after he first comes under suspicion, both of his eyes are brown. (00:12:02)
Continuity mistake: When Cody comes out of the turbolift with Slick, he's carrying a plain white helmet instead of his regular customized one. (00:00:49)
Character mistake: When Commander Cody says "We'll get right on it", Obi-Wan responds with "No, Captain", calling him by the wrong rank. (00:05:18)
Continuity mistake: When Anakin and his men rush to aid Obi-Wan in the south tower, about half a dozen droids marching inside separate the two Jedi. Anakin begins cutting them up; one of the droids is already crossing the threshold. But when Anakin finally reaches Obi-Wan, the droid who has already entered is suddenly no longer present. The hips and legs falling in before Anakin rushes in come from another droid just outside the door.
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.