Revealing mistake: When Grievous falls to the ground as the Gungans throw plasma balls at him, the lance General Tarpals stuck through his torso blatantly clips through the ground. (00:15:03)
Revealing mistake: When one of the Mandalorians snags Ahsoka's right wrist with a cable, it clips straight through her head and neck in the process. (00:15:46)
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.