As the two couples are first seen waiting at the door to the Decker's house, there is a tree behind Janet. After the cut-away, as Tom opens the door and greets them, the tree has moved five feet to it's right, and is much smaller.
Trina tells Susan that Gail's husband isn't 'on board' with the open marriage concept. It is later established that Gail is single.
The airplane seen from the outside in flight is a Boeing 767 which wasn't yet in service in 1976.
In the scene showing the Sears Tower you can see the twin antennas on the top of the building. The antennas were not added until 1982, but this show is set in 1976.