Old Friends Not Forgotten - S7-E9
Revealing mistake: When Anakin says to Obi-Wan that if they do things his way they can help the people in the city now, his hair is blowing in the wind, and the locks by his forehead are obviously clipping into his head. (00:02:42)
Old Friends Not Forgotten - S7-E9
Revealing mistake: When Rex hands Anakin The Box with Ahsoka's lightsabers, his right thigh armour piece briefly clips through his kama twice. (00:12:58)
Revealing mistake: When Ahsoka and Rex walk onto the bridge of the Star Destroyer, in one shot the right part of his kama is partially transparent, revealing his thigh armour beneath it. (00:08:05)
Revealing mistake: When R7 goes to plug in to the socket to look up information on Fives, he moves right in front of one arm of a droid charging station in the room which should be in front of him. (00:16:32)
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.