When Walter is driving into Stepford, both Joanna and the kids disappear from the car, then re-appear again.
At the picnic, background actors change hairdos from shot to shot. They look different outside than when they are square dancing.
When Roger, Bobbie and Joanna are at the bottom of the stairs in Sarah's house, their positions change between scenes.
When Joanna enters the house looking for her children, her blouse is unbuttoned, then is buttoned again in a subsequent shot.
When Claire drives Joanna through Stepford, her seat belt disappears and reappears between shots.
When the family is driving to Stepford, Pete says "But why are we moving?". Kimberly can be clearly seen mouthing his line before saying "to Conneticut?"
Moments before Walter drives to up to the Stepford community gates, the scenery outside his window shows cars passing by him even though he is driving on a one-way street.
A dark clothed crew member (cloaked?) raising an arm is visible in a reflection just before Matthew backs into a panel.
Boom mic reflection visible in window over door in scene when Walter replies, "When I get back."