Continuity mistake: At the beginning of the queen's audience with Count Dooku, when he says, "But I was invited - by your faithful advisor. He's concerned over how attached you've become to Skywalker," there is a shot of Dooku walking up the room, then it cuts away to the queen and Atai Molec (the advisor in question), and when it cuts back to Dooku, he walks through the same part of the throne room as in the previous shot of him. Take note of a screen on the side of the room - it's in the same position both shots. (00:05:35)
Continuity mistake: When Queen Miraj gets angry with Atai and Count Dooku, she throws her glass to the floor and the sound of shattering is heard. But after Anakin arrives, the glass is lying on the floor unbroken. (00:09:10)
Continuity mistake: When Anakin is cutting through the door of the Kadavo facility after Master Plo's forces have arrived, the position of the hole he's making is different from when it was previously shown. When he finishes cutting the hole and kicks in the cut-out section of door, the hole is back in its original position. (00:14:25)
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.