Revealing mistake: When Grievous falls to the ground as the Gungans throw plasma balls at him, the lance General Tarpals stuck through his torso blatantly clips through the ground. (00:15:03)
Other mistake: When Anakin is fighting some commando droids he suddenly has two lightsabers, one that he is actively using and another on his belt. (00:06:11)
Character mistake: When Queen Neeyutnee thanks Jar Jar at the end, she erroneously refers to him as "Senator" Binks (he's a representative). (00:21:46)
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.