Continuity mistake: In the opening titles (for series 1-3), Stan runs outside and picks up a newspaper. He reads the headline and throws it back on the ground. But by the time Stan drives off, the newspaper has disappeared. (00:00:10)
Continuity mistake: The window partway up the Smith staircase vanishes after the first season.
Continuity mistake: The vent-hood in the kitchen above the stove/island is always disappearing and reappearing. Sometimes it's there, sometimes it's not. The handles on the fridge/freezer also keep changing sides. Sometimes they are on the left, sometimes they are on the right. And the living room is always changing sizes. Sometimes they open the front door and the couch is right there, and sometimes it's several feet away from the front door.
Chosen answer: It is just the decision of the creators to do so.
MasterOfAll