The Citadel - S3-E18
Continuity mistake: Just after Ahsoka says "clear" when they're sneaking around the prison, her Padawan braid is missing again. (00:13:40)
The Citadel - S3-E18
Continuity mistake: When the commando droids enter the cell where Piell's officers were being held, the damage on the door from Anakin stabbing a commando droid with his lightsaber through it is gone. (00:19:20)
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)
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.