Continuity mistake: During the kitchen scene when Janet is eating the egg salad sandwich, her yellow napkin is tucked in her blouse and then disappears. She then tucks another yellow napkin into her blouse.
Diamond Jack - S7-E3
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. (00:25:00)
Jack Bares All (a.k.a. Oh, Nurse) (2) - S6-E2
Continuity mistake: In the scene where Terry puts on the gag glasses, notice how the black circles under her eyes get larger after the shot changes.
Grandma Jack - S8-E7
Visible crew/equipment: Notice the scene when Terry comes out of the kitchen. As the kitchen door begins swinging shut, a mysterious hand (which belongs to a crew member) suddenly appears from offstage to keep the door closed. (00:07:10)
Good Old Reliable Janet - S3-E2
Continuity mistake: When Janet and Mrs. Roper are found nude behind the bushes, Larry leaves to get towels. However, the yellow towel suddenly appears briefly on screen several seconds before Larry returns.
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.
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.
Strangers in the Night - S6-E12
Character mistake: The whole idea of the serenading scene was Larry was supposed to hide and sing for Jack while Jack would be shown in the yard alone and pretend to be the one singing, but the yard was brightly lit and Larry was right behind Jack and both were very shown with nothing to hide Larry.
Lee Ain't Heavy, He's My Brother - S4-E21
Character mistake: Jack's older brother Lee comes for a visit. In the second season when Jack's Uncle visits, both make note of Jack being his only nephew. Lee would be his other nephew.
Suggested correction: Although it's not stated in the show, theoritically speaking, it WOULD be accurate if Jack and Lee were HALF-brothers (with same father since both last names are Tripper) and Fremont was the brother of Jack's mother, then Jack COULD be his only nephew as Lee would have NO relation to Fremont.
I submitted the above correction based on a theory that would make it possible, although in reviewing one of the scenes involving Jack's Uncle, I realise this would not be the specific case. Uncle Fremont most likely is the brother of Jack's father (not mother like the theory) due to the fact when he tells Mr. Roper that Jack's father is happily married but that doesn't stop him from having "fun" with a friendly nudge. He most likely would not say that if it was against his own sister. So yes, the theory didn't apply in this specific situation. Instead, the situation here is during the Roper early seasons, the episode came out with the Uncle and a year or two later (during the Furley era) Lee was intruduced. Neither were ever mentioned in any other episode so it could be simple inconsistent screenwriting.
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.
Continuity mistake: When Furley enters The Apartment at the start of the party, the outside weather is crystal clear. Less than 2 minutes later, Furley tells "Austin" the fog is very heavy and the airport is closed down. Then upon opening the front door, tons of thick fog is all over outside. While fog can roll in very quickly, how could Furley even bring up the fog in the first place when it was not there at all when he walked in?
Grandma Jack - S8-E7
Other mistake: Larry entered Jack as "Grandma Tripper" to win the contest since the contest was only open to women. However, since Larry admits he didn't know there would be a bake-off and thought a winning check would just be mailed, then there's no reason to of entered Jack as "Grandma." He easily could have just entered Janet or Terri using Jack's recipe.
Double Date - S3-E1
Visible crew/equipment: At the start, while Mr. Roper is in the kitchen the faucet sprays water and the doorbell rings. Note the cabinets and wall behind him. When Roper walks out of the kitchen, as the swing door is open we can see the kitchen, and the upper cabinets are split apart with the wall missing, so the area behind the kitchen set is visible. (00:00:50)
Visible crew/equipment: When Mr. Roper gets the idea that it's Jack's diary, Jack tells him not to worry and it'll be their own little secret, then in the next wide shot we can see the bank of set lighting at the bottom of the screen. (00:21:15)
An Anniversary Surprise - S3-E20
Visible crew/equipment: At the end of the episode when Jack falls back off the chair, a boom mic is very visible above him. (00:25:15)
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).
Revealing mistake: At the end where Janet and Jack are in the bathroom (off camera, as he is confessing why he took the car), - you can plainly see that the door to the bathroom is a breakaway door compared to the other episodes, - and even in that particular episode. It's changed in the middle of the episode.
Continuity mistake: When the painting of Jack is hanging in the Regal Beagle, the painting is just legs, and not Jack's whole body. In some shots the top of the painting can be seen.
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.
Visible crew/equipment: When Janet's father hands Jack the big gift, a microphone appears for a second at the top of the screen. (00:03:15)
