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)
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)
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: 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.