Continuity mistake: After Jack brings the puppy over to the girls, the box he brought the puppy in was placed on the table behind their couch. At one point, it is knocked over, and you can see it falling down to the floor (past Janet). In the next shot, 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."
Revealing mistake: In the scene where Jack and Janet are sitting on the sofa with the doctor, Janet offers the doctor some nuts from a bowl. Then take a look at Janet's face. She begins to laugh so she lowers her head and the camera immediately shoots over to Jack who realizes the goof and adlibs something to give Janet time to get into character again.
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 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.
Other mistake: After Jack leaves the apartment with Mr Angeleno, Janet and Terry 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.
Visible crew/equipment: In the episode "A Camping We Will Go", when Lana and Mr. Furley show up to the Cabin unexpectedly, watch closely when Lana tries to sit next to Jack on the couch.if you look through the window by the flag hanging on the wall you'll see Larry's girl, Laura, waiting outside by the front door (in her bright pink outfit) waiting to make her entrance.
Other mistake: Jack goes into the kitchen with Janet to get "Jim Bob" Furley a glass of brandy. When Jack returns from the kitchen to hand Furley his drink, Jack trips and spills the drink all over Furley, causing Furley to give the Partnership to Jack's Navy pal, Bill. Furley then excuses himself and heads to the bathroom to clean up. But Furley isn't supposed to have met Jack before, so he wouldn't know where the bathroom is. He goes right to it anyway without asking where it is. Lucky Bill didn't notice.
Continuity mistake: In the scene where Jack's uncle first walks in the house to greet Jack, his uncle says, "I thought I'd drop in and visit my favorite nephew." Jack then responds with, "Well I'm your only nephew." But in Season 4, Episode 21 ("Lee Ain't Heavy, He's My Brother"), we meet Jack's brother, Lee Tripper.