Revealing mistake: The lights around the stage reflect in Jim Walsh's sunglasses a few times.
Will the Real Jack Tripper . . . - S2-E21
Character mistake: When the angry father comes to the door at the end of the episode looking for Jack, Mr. Roper opens the door and after the father asks "Jack Tripper?", Mr. Roper responds "Yes" and then gets chased around by the father assuming Roper is Jack. Why would Roper respond "Yes" anyway? He's not Jack and would never say he was - just scripted as a way to get chased around but in a real situation, he would have no reason to answer that way.
Days of Beer and Weeds - S2-E22
Visible crew/equipment: Reflections of the monitors and other equipment are visible in Janet's glasses at the end.
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.
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)
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.
Good Old Reliable Janet - S3-E2
Character mistake: At one point in this episode, Jack is chatting with Larry about the nude beach and in frustration says "all the best looking girls were wearing moth-eaten blankets" right in front of Mr. Roper as well as showing interest in seeing Janet without her clothes also in front of Mr. Roper. Jack is supposed to pretend he's gay when Mr. Roper's around him. Roper luckily didn't pick up on it this one episode either time (rather showing concern for his wife).
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)
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.
Suggested correction: True, there's no logical way that Jack could have sent the telegram. But Janet and Chrissy were upset about it - when someone's upset, they don't always think in a logical way.
Or he could've called Larry and had him set it up but I don't think he ever used the phone to call anyone.
Plot hole: At the party, when Jack tries to suddenly stop many guests from eating his tainted cake, he knocks away the cake from many people to prevent them from eating it - but at least two minutes or so have gone by since the guests were given the cake slices so most likely, many (if not all) the guests would have tried the cake within those two minutes. And yet somehow Mr. Roper was the only one who is revealed to have actually tried the cake in all that time.
Visible crew/equipment: When Jack opens the medicine cabinet to get the shaving cream, in the mirror's reflection you can see the camera as well as where the set ends.
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)
Visible crew/equipment: A large shadow of a crewman's shoulder is visible on the wall behind Larry after Janet exits to the kitchen. (00:16:37)
Revealing mistake: Studio lighting on Janet and Chrissy's tent at night, in the middle of the woods. (00:21:12)
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.
Handcuffed - S4-E18
Plot hole: Chrissy's policeman cousin informs the roommates that there was a complaint from a neighbor about her husband on his roof watching the girls dance in The Apartment with binoculars. However, when the girls were dancing, the front door was fully closed and the very thick drapes were fully closed so how could such a complaint exist?
Handcuffed - S4-E18
Continuity mistake: When Jack is eating cake and Chrissy is under the table her the sleeve of her right arm is down. When Jack and Chrissy exit the kitchen into the living room, both of her sleeves are down.
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.
Upstairs Downstairs Downstairs - S5-E1
Visible crew/equipment: Shadows from the booms are quite visible in varied takes, along with false ceiling in the living area of their apartment.
Visible crew/equipment: Obvious shadows from booms, overhead, and crew members reflect off the walls while Jack and Larry are talking, and Larry looks like he's either reading from cue cards or following an off-camera director for his blocking moves.
