The Citadel - S3-E18
Continuity mistake: Just after Ahsoka says "clear" when they're sneaking around the prison, her Padawan braid is missing again. (00:13:40)
The Citadel - S3-E18
Continuity mistake: When the commando droids enter the cell where Piell's officers were being held, the damage on the door from Anakin stabbing a commando droid with his lightsaber through it is gone. (00:19:20)
The Citadel - S3-E18
Continuity mistake: When Ahsoka asks Master Plo if she can speak with him, her Padawan braid (the beaded chain hanging on the back of her head below her right horn) is missing. (00:04:05)
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.