Continuity mistake: When Yoda arrives at Korriban, the wings of his starfighter open up when he flies down to the planet. However, in the second shot of him flying down, the wings are closed. (00:01:10)
Continuity mistake: While Anakin and Obi-Wan are talking about Rush Clovis, a droid head Anakin is working on disappears from a table between shots. (00:07:30)
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.