Audio problem: When Samantha and Darrin are at his ex-girlfriend's house for dinner, Samantha is doing all kinds of things to sabotage the dinner, even making a gust of wind come through the front door thus blowing the ex's wig off. The ex girlfriend is screaming but the screams don't match up to what her mouth is doing.
Other mistake: While Gladys Kravitz is standing in front of her house, which is across the street from the front of the Stephens' house, Gladys sees the Stephens' backyard just as Sam zaps a swimming pool and then zaps it away, but since the Stevens' backyard is located behind the house where the living room and dining room are, this would be impossible even for nosy Gladys Kravitz.
Continuity mistake: When Dave sees Darrin's ad for the community meeting, he suggests a couple of changes, one of them being to add a line saying "and bring your friends". But the ad was shown in close-up shortly before Dave's arrival, and there was already a line saying "Be sure and bring all your friends".