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: Larry gives Cindy and Janet gifts to thank them for letting him live with them, so Janet gives him a kiss. However, three seasons later in the episode "Friends and Lovers," Janet kisses Larry at her wedding to which Larry says "Janet, that's the first time you've ever kissed me."
Continuity mistake: This is such an obvious mistake. In the scene where Jack, Janet and Terri are on the air for the cooking show, Janet holds up the silver bowl but there is no parsley in it. Then in the next shot the bowl is full of parsley. How is it possible that it got filled up in a matter of two seconds.
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.
Continuity mistake: When Mr. Roper tries to explain to the girls and his wife what he was doing in Jack's bedroom, watch over Mr. Roper's left shoulder on the shelf. The number of yellow cans on the shelf keep alternating between one and two cans throughout several minutes.
Continuity mistake: When Jack is introduced to Mr. Roper for the first time, the girls just say he's their landlord (no name given yet), however after Jack's talk in the kitchen with Chrissy (in which still no landlord name given), Jack storms out and starts out shouting "Look here Mr. Roper..." He still should not have known the landlord's name at this point.
Continuity mistake: The kitchen changes from episode to episode, when viewed from the living room. If the set is going to be used the full kitchen is visible when viewed. But if it is not being used, often the set is pushed closer together. This is most evident when an actor is coming out of the kitchen the stove and the attached cupboards are practically at the door, providing just a bit of space for the actor to stand before making their entrance. Other times a wall is simply pushed up beside the door with the kitchen wall paper on it to appear to have continuity. This is only seen when the shot allows the kitchen set to be obscured.