House - Apartment Door
When, at the end of the first season, Kameron visits House at his home, he opens the apartment door which is in a corner with another door (to the bathroom I guess) at the right side. But in the series when Wilson waits in front of the door (they agree Wilson will wait if House's stethoskope is hanging at the door knob, you see the apartment door is just behind the house door at the left side. The layout is strict impossible. When House eventually opens the door and both enter, the whole section of the apartment has changed, in comparison to the layout in the first season.
I haven't noticed House moved to another apartment, so this is a true major slipup.