Continuity mistake: When Mace and Ponds have the exchange that ends with the Jedi General dropping "I have a bad feeling about this", Ponds' kama (kilt) has a blue border instead of the usual maroon to match his armour. (00:08:43)
Continuity mistake: When the Chancellor is being briefed on The Zillo Beast, Rex's kama is missing. (00:14:12)
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.