Continuity mistake: When Muriel Goldman dies, everyone gathers up around her. When that happens, you can see Tom Tacker and Meg in the background, although they both disappeared a few scenes earlier. And than when everyone gathers downstairs, they do a body count and discover Tom and Meg are missing and only then are they found.
Continuity mistake: When the children are singing, "So Long, Farewell", as they are going upstairs they pass a picture of Lois' parents hanging along the middle of the stairwell. When they get to the top of the staircase, we see the exact same picture, but now at the head of the stairs.
Continuity mistake: In the part where Meg is asking Brian about God, and Brian answers he is an atheist, the camera pans out quickly, and we see an entrance to the kitchen with an air vent above it. Eventually the family discussion with Brian continues, and you see that the air vent is now gone.