Stupidity: Xander really has no reason not to believe Buffy when she tells him that Miss French is evil/a giant bug. There is an in-universe explanation of pheromones affecting him, but that's flimsy at best. He's already dealt with vampires and witches and body-swapping, and there has already been a murder around the time Miss French showed up... the fact he doesn't believe Buffy seems very forced and contrived.
Stupidity: The master goes through all the trouble of making Luke the vessel so Luke can go and kill several people and risk getting the attention of the slayer, when all he had to do is have his minions go out and kidnap a bunch of people and bring them to him and he could've fed off them himself.
Chosen answer: He had larangitis and because of the shooting specifications they had to have him shoot the scene with a bad throat, they then re-recorded his lines and dubbed them. At various points during the scene and indeed the episode you can see him touch his throat or rub it.