Revealing mistake: Any time a main house light or a lamp is switched off, it is still bright from the studio lights.
Continuity mistake: In the very first episode, there is a silver light fixture hanging from the ceiling in front of the kitchen. The light fixture appears and disappears throughout the first season.
Character mistake: After Jack (who's hiding from Chrissy's mother) ducks behind the couch holding a mop, Chrissy's mother comes out of the bathroom (which from the camera angle shown is in a straight line facing the back of the couch where Jack is) and walks to the right and all the way around the couch and sees the mop behind the couch and picks it up without seeing Jack at all. From where the bathroom is, she would have just needed to walk straight for the mop and seen Jack (yet she missed him).
Continuity mistake: After Jack brings the puppy over to the girls, the box he brought the puppy in was placed on the back end of their couch. At one point, it is knocked over, and you can see it falling down to the floor (past Janet). In a few shots, it's back on the table, and none of the characters were in a position to pick it up to place it back there.
Continuity mistake: While Jack is talking to the police officer about the missing rent money, they are standing in front of the TV set and the front door is open. When it cuts back to Jack and the officer, over Jack's shoulder the front door is closed, but when the officer turns to leave the door is open again.
Revealing mistake: When Jack and Janet are in the entry looking around for burglars, their voices are over-amplified, indicating they are in a sound studio.
Answer: Richard Kline and Don Knotts were supporting cast members, who could be replaced at anytime. It was John Ritter, Joyce DeWitt and Suzanne Somers who were the star vehicles they relied on to carry the show.