A piano is included in the Stephen's living room where none has been before, simply so a couple of jokes about it could be included in the script.
In the opening, Samantha hears Darrin close the front door while standing at her bedroom door. She immediately rushes over to the front window less than 10 feet away, but Darrin has not only gone 30-40 feet to the car but has gotten in, started it up and is already backing out of the driveway. Not realistic timing, but necessary for the unimaginative director (he would have been better off not having the front door closing sound effect) to get to the part where Tabitha pulls down the breakfast tray.
Just after Samantha is talking on the phone with Darrin while she is floating, the green board (very close to the same color as her dress) can be seen that Samantha is laying on.