Continuity mistake: When Obi-Wan and Mace retrieve the stolen holocron at Black Stall Station, the device is shown as completely closed and back in a cube configuration. This is despite the fact that the previous episodes of the arc established that Jedi holocrons can only be opened by Jedi, and it was last seen partially opened after Bane inserted the data crystal, with no contact with any Jedi between Bane's escape and the arrival at his hideout. (00:15:08)
Continuity mistake: The opening scene after the narration shows the end of "Cargo of Doom" again, focusing on the clone troopers, but the details are different, including Anakin and Ahsoka disembarking the shuttle with R2 and some of the clones removing their helmets before leaving, when that didn't happen in the previous episode.
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.