The stem of the duplicating plant Dr. Smith was showing to Will is actually a vent pipe for a clothes drier.
The control panel of Maureen's clothes washing machine seen at about 4:50 has the words "speaker amplifier" and controls labeled "gain" which would have nothing to do with cleaning clothes. The panel is obviously part of a PA system or some other type of audio amplifier.
Though a stowaway aboard ship, Dr. Smith nonetheless has his own shaving kit and shaving lotion.
Will, Don and Penny all know about Dr. Smith losing his deutronium, even though Smith has only just revealed that information to Prof. Robinson in a private talk with no others around.
In other episodes, a point is made over the Robinsons continuing the initial journey to Alpha Centauri, but here they only talk of returning to Earth.
The Robinsons talk of soon leaving the planet and returning to Earth. This isn't directly possible since the they don't know where in the galaxy they are in order to determine which direction to take.
When Professor Robinson first is entangled by the plant vine, but before he is pulled into the quicksand, a laser pistol is clearly shown attached to his lower body, yet he yells to Dr. Smith for help when he could have easily just shot the vine.