Continuity mistake: After the droid advance force has split up to hunt down Master Yoda, the Jedi Master and his clone escorts attack each group on their own. When Yoda ambushes his group, it has six battle droids; but he is seen finishing only four of them. The remaining two just vanish from the scene. (00:10:10)
Continuity mistake: For one shot near the start, the lightsabers on Ventress' belt are behind the buckle, but in all other scenes, they are hung in front of it. (00:03:56 - 00:04:38)
Continuity mistake: After Yoda cuts a hole in one of the droid battle vehicles, and throws the droid out, the following scene returns to Asajj and the Toydarian king. When their backs are shown, Asajj is standing close beside the king, but in the very next shot (showing them from the front), she is obviously a few feet farther from him. (00:17:08)
Revealing mistake: When the battle droid commander orders the tanks to stop because they can't go any further into the forest, the fingers of the droid nearest to the camera clip through the wall of its tank. (00:07:49)
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.