Continuity mistake: When Obi-Wan duels Grievous on the bridge of his Arquitens cruiser, the cyborg General slashes up several computer consoles with some of his four lightsabers. Later, when Obi-Wan sends two clone officers to the bridge to try to detach the ship, the consoles are in the background with no visible damage.
Continuity mistake: When Ahsoka tells Rex to go to the briefing, the rectangular pouch thing attached to the right side of his breastplate is missing. (00:09:24)
The Phantom Apprentice - S7-E10
Continuity mistake: During Ahsoka and Maul's duel, just before she kicks him through a wall, he's pushing her down with his double-bladed lightsaber while she's resisting with her two lightsabers. In a shot from behind her, she has one blade on each side of the handle of his lightsaber, but a close-up of her shows both of her blades on the same side. (00:20:19)
Old Friends Not Forgotten - S7-E9
Continuity mistake: When Cody is telling Obi-Wan about the tactical droid remaining out of sight, the armour accessory he wears on his back is missing. (00:02:05)
Continuity mistake: Rex's kama is missing in a long shot of him and Ahsoka during the hangar bay firefight.
Continuity mistake: When Ahsoka runs out of the medical bay, Rex's kama is missing. (00:01:18)
Continuity mistake: Throughout the episode, clone troopers are wearing armor with four different types of markings; some have none, some bear the markings of the Coruscant Guard, some bear the markings of the 332d Company (Ahsoka's unit), and some bear the markings of the 501st Legion. However, at the end of the episode, the memorial in which clone trooper helmets are placed on their graves, only helmets with no markings and helmets with 332nd markings can be seen, none from the 501st Legion or Coruscant Guard.
Continuity mistake: When Yoda arrives at Korriban, the wings of his starfighter open up when he flies down to the planet. However, in the second shot of him flying down, the wings are closed. (00:01:10)
Nomad Droids - S4-E6
Continuity mistake: At the end, when R2-D2 and C-3PO are reunited with Republic forces, Master Plo asks Commander Wolffe if he knows these droids. However, Plo previously encountered R2 in both "Rising Malevolence" and "R2 Come Home" (the latter episode having the same error).
Continuity mistake: When Rex is giving orders to the heavy cannons after the super-tanks emerge, his kama is missing.
Continuity mistake: Rex has a bandage on his left arm for the entirety of the episode after getting injured in the previous one. After Aayla destroys the probe droid, in a shot of her, Ahsoka, Rex and Bly walking over to the destroyed droid, the bandage is missing. (00:07:11)
Old Friends Not Forgotten - S7-E9
Continuity mistake: When Ahsoka and Rex watch from their gunship as another one is shot down on the approach to Sundari, his kama is missing. (00:18:24)
The Phantom Apprentice - S7-E10
Continuity mistake: When Ahsoka, Rex and Bo-Katan are watching the evacuation, Rex is holding his helmet under his left arm. When the three of them are going inside as Bo remarks that the occupation cannot last much longer, Rex is suddenly empty-handed. (00:13:27)
Old Friends Not Forgotten - S7-E9
Continuity mistake: When Bo-Katan storms the throne room, the pistol she's holding in her right hand is kicked out of her hand by one of the supercommandos. However, when she goes to fight Almec, there's a pistol in her right holster despite her not having had time to retrieve the weapon. (00:23:30)
Continuity mistake: When Anakin and Ahsoka are pinned down behind cover by the sniper droids, the damage the droids' blaster shots on the walls the Jedi are hiding behind changes whenever it's shown. (00:14:26)
Continuity mistake: When Krell is getting in Fives' face about who is going to take the blame for Fives and Jesse disobeying orders, the markings on Jesse's armour are missing. (00:21:21)
Continuity mistake: When Tamson is mocking Lee-Char about asking Nossor Ri for help, Ahsoka has her lightsabers on her belt even though she was disarmed earlier when she and Lee-Char were captured. (00:13:16)
Continuity mistake: R2's squad of battle droids has three members. Yet in the climactic fighting of the previous episode, one of the droids was shown getting destroyed.
Continuity mistake: After Obi-Wan's group has abseiled down the wall of the prison, two of the clone troopers in armour are carrying Separatist blasters instead of the Republic-issue blasters they had before and after. (00:09:38)
Continuity mistake: When the possessed Ahsoka declares that Anakin will be forced to kill her and Force-pulls one of her lightsabers into her right hand, said hand changes positions between the wide shot and the following close-up. (00:12:28)
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.