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)
Senate Spy - S2-E4
Continuity mistake: When Padmé accesses the holo-table with the droid factory plans, she puts her clutch purse on the edge of the table. After she guesses the correct password, the purse is absent from a shot facing her as the hologram activates, but present in the next shot. (00:15:56)
Continuity mistake: When Count Dooku is speaking to Riff Tamson via hologram, the clasp of Dooku's cloak is missing in one shot. (00:00:57)
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.