Continuity mistake: When Ahsoka is boarding the rescue gunship, her normally green lightsaber has a blue blade. (00:18:00)
Continuity mistake: When the Chancellor is being briefed on The Zillo Beast, Rex's kama is missing. (00:14:12)
Continuity mistake: When Dooku Force-throws some cutlery and dishes at Anakin, he gets a fork and a knife stuck in his prosthetic arm, but when he pulls them out, there is no damage to the protective glove he wears over it.
Continuity mistake: When Wrecker pulls Rex off Crosshair after the former attacks the latter for making insensitive remarks about Echo's presumed death, Rex's kama (kilt) is missing in the close-up shot. (00:14:10)
On the Wings of Keeradaks - S7-E3
Continuity mistake: When several of the clones are climbing up the ventilation shaft, Rex's kama (kilt) is missing. (00:04:17)
Continuity mistake: When Jesse and Crosshair are glaring at each other onboard the gunship, Kix has his right arm raised to grab hold of one of the handholds. When Jesse starts forward after Crosshair calls him a "reg", Kix's arm is suddenly down, but when Cody tells everyone to cut the chatter, Kix's arm is raised again. (00:07:00)
Continuity mistake: When Hunter, Wrecker and Jesse are holding off droids at the cyber centre's front entrance just before they're forced to retreat inside the facility, Jesse's kama (kilt) is missing when he briefly emerges from behind a stack of crates to fire at the droids. (00:21:12)
Continuity mistake: The old photo Rex has of himself, Cody, Fives and Echo shows Cody and Rex wearing Phase II clone armour. This is even though the picture must have been taken before the events of season 3's Citadel arc that included Echo's presumed death, and Phase II armour wasn't introduced onto the show until season 4.
Continuity mistake: After Rako Hardeen orders a drink at a cantina, when the bartender pulls a bottle out from under the bar, it's corked when he takes it out and raises it to pour, but the cork is suddenly missing when he's pouring, although he didn't stop to take the cork out. (00:04:00)
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)
Slaves of the Republic - S4-E12
Continuity mistake: When Ahsoka threatens Queen Miraj, she's holding her primary lightsaber. When she's shocked unconscious and falls to the ground, it's her smaller shoto that rolls onto the floor. (00:14:44)
Nomad Droids - S4-E6
Continuity mistake: During the incinerator scene, there are two RA-7 protocol droids seen. When the first one attempts to run from the incinerator, the one behind it has vanished, only to reappear in the next scene. (00:20:24)
Continuity mistake: While training Savage, Dooku uses the Force to levitate a number of stone obelisks. While he's doing that, there's a closeup of Savage where an obelisk that is shown floating in the preceding and successive shots is visible behind him not floating. (00:03:36)
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)
Continuity mistake: When Padmé and Bail are walking down the hall after the Senate vote, the top of her right sleeve is miscoloured, with the purple section extending further down than it should on that outfit. (00:21:02)
Continuity mistake: When Ahsoka suggests that Chairman Papanoida's daughters might be being held on the Trade Federation ship blockading Pantora, she refers to it as a planet. "Trespass" established that Pantora is a moon. (00:06:47)
Continuity mistake: When Krell jumps out of the tower window, his blue lightsaber is in his right hand, but when he lands, his sabers have swapped hands, with the green one in his right instead. (00:12:21)
Continuity mistake: During the opening briefing with Palpatine, Mace Windu and Yoda, Luminara Unduli attends via hologram. But during the shots where Obi-Wan is explaining the attack plan, and the shot over the shoulder of Palpatine's hologram looking at Obi-Wan and Ki-Adi-Mundi, Luminara is shown as being physically present (i.e. not a blue, translucent hologram). After Palpatine, Mace and Yoda sign off, she's back to being a hologram. (00:02:00 - 00:02:30)
Continuity mistake: After the first encounter with the zombie Geonosians, it's shown there are four clone troopers in the group when they retreat. When Obi-Wan orders two of the clones to try and head back to the surface to get reinforcements, there are suddenly several more clones present. (00:11:32)
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.