The General - S4-E8
Continuity mistake: When the 501st are pushing towards the Umbaran Airbase, most of the injured clones Kix drags away are missing their 501st markings, then in the next shot, they have the markings. (00:01:28)
The General - S4-E8
Continuity mistake: Pong Krell's goatee is missing at the end, when he accepts Rex's report that the base has been successfully captured. (00:21:02)
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.