Deliberate mistake: At the beginning, Asajj Ventress is wearing an outfit with a long skirt. When she is rescued from her damaged starfighter by the scavengers, the skirt has vanished. This was done because the animators found the skirt too difficult to animate. This is also why she receives a brand new outfit that never had a long skirt later in the episode. (00:08:20)
The Citadel - S3-E18
Continuity mistake: When Ahsoka asks Master Plo if she can speak with him, her Padawan braid (the beaded chain hanging on the back of her head below her right horn) is missing. (00:04:05)
Continuity mistake: While training Savage, Dooku uses the Force to levitate a number of stone obelisks. While he's doing that, there's a closeup of Savage where an obelisk that is shown floating in the preceding and successive shots is visible behind him not floating. (00:03:36)
Continuity mistake: When Baron Papanoida walks into Jabba's court, his half-cape is missing. (00:10:24)
Answer: I guess that 1) the King assumed that Ventress' droids were meant to capture the Jedi, not kill him outright. When she yells at OOM-224 to shoot Yoda, she evidently failed that expectation. 2) Sending out a bunch of Droidekas after it is plain that Master Yoda has clearly won is not a sign of good sportsmanship, either.