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.
Revealing mistake: In the episode "Out on a Limb", Jack leaves Terri in Mr. Townsend's office lobby to keep the secretary busy while Jack is supposed to climb a tree outside up to the window and move from window-to-window to get to Townsend's office. If you look at the window behind Terri and the secretary, you'll see Jack magically walk through the brick wall to get set up for the tree climbing scene.
Visible crew/equipment: After Jack talks to Larry about the magazine quiz Jack asks Janet if anything's been bothering her, and just as Terri walks out of the kitchen holding the two cans, a crew member's hand can be seen behind Terri placing a bag on the counter.
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.
Visible crew/equipment: Near the start of the show, money comes out of the ATM by error. You can see the money waiting in the slot and the crewmember's hand pushing the money out sporadically. It also comes out sideways sometimes, sometimes not and out of different parts of the slot.
Revealing mistake: As Terri exits Jack's bedroom to stall the diamond thieves, you can momentarily see into the girls' bedroom. The wall that should be between the girls' bedroom and Jack's bedroom is missing and the floor and blue curtains of Jack's bedroom set are visible. Someone's arm wearing a light colored shirt (John Ritter's sleeve is brown and Joyce DeWitt's sleeve is black) can also be seen moving.
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.
Other mistake: After Jack leaves the apartment with Mr Angelino, Janet and Terri are sitting on the couch trying to figure out a way to get money for Jack. When Janet gets up you can see the bathroom door start to open behind her but there is no one else in the apartment to open it.
Plot hole: Janet and Chrissy accuse Jack of making the fake BBQ telegram. However, Jack was at the apartment the whole time, never left once until after he found out Lori was the one who made the telegram. If it was Jack who made the fake telegram, he would have had to leave the apartment to do that.
Character mistake: Maria (Felipe's cousin) supposedly didn't understand any English (only single words like "yes") and would answer (in Spanish) "Que?" after a full sentence of English was spoken to her. However, in front of Jack, one of the girls said to Maria "Tell Jack the new English word you just learned" and Maria understandingly responded "No." There's no way that Maria could respond correctly to that question since she didn't understand any other English sentences throughout the episode.