Character mistake: Janet is waiting for Larry to deliver her new car and states to Jack that this is her very first car. But 5 episodes earlier in S8-E13 "Itching for Trouble" when Moose asks if they had a football that needed air, Janet says no, but one of the tires on her car are low.
Continuity mistake: When Jack is on the phone explaining why he can't go into work again because he's really hiding at home from the bully Max, there is a chair wedged up against the front door. Larry knocks and when Jack is sure it is indeed Larry, he unlocks and opens the door without the chair in the way any more.
Other mistake: In the Regal Beagle, Jack addresses Marsha as Ms. White although later in the episode her last name is revealed to be Whitewood. In addition, how could Jack have known her last name anyway? Marsha only said her first name in front of Furley and the trio early in the episode and presumably she hasn't talked to the trio since she left with Furley out the door.
Suggested correction: In addition to the other correction (occurring 00:07:25), this correction addresses the second part of the mistake, as to how Jack knew Marsha's last name. Earlier, when Larry tells the trio that Furley was fine with him being late with the rent, Janet states, "He's been wining and dining her for 3 days" (at 00:06:15). So at any time during those 3 days (offscreen), Furley told the kids about his dream girl, which would have included her last name.
Suggested correction: No Jack does say, "Hello Ms. Whitewood!" While they are shaking hands. Time stamp in the episode when this occurs is at 7:25.
She Loves Me, She Loves Me Not - S8-E2
Visible crew/equipment: Just before Larry falls off the boat into the lake, a shadow of a boom mic is visible.
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.
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).
Diamond Jack - S7-E3
Character mistake: At the roller skating rink, Jack looks at the menu on the wall in the snack bar while ordering two sodas and says "What are they... 50 cents a piece?" However, only food items and candy are listed on the menu and drinks are not mentioned at all. How did he know the price?
Character mistake: The undercover cop who first mistook Chrissy for a hooker, first admits he was wrong about the way he judged her. Then upon finding out that Jack lived with her and Janet, he came on to her instantly thinking she's "easy" once again. However, the cop was already informed by Mr. Roper that Jack was gay at this point, so the fact that a "gay" man living with women should not have changed the cop's mind of how to view Chrissy the way he did.
Continuity mistake: In this episode, Larry is a really poor sounding singer, but in the episode "Strangers in the Night (6-12) " in the previous season, Larry sang perfectly without any problems.
Character mistake: When Jack tells Mr. Furley his brother is in town (before mentioning it was a "twin" brother who Jack made up), Mr. Furley gave a puzzled look and said "Your brother?" which appeared as if he didn't know Jack had a brother. However, in the previous season, Mr. Furley briefly met Jack's brother Lee, so he already should have known Jack had a brother (even if were to turn out to be a different brother).
Jack Goes to the Dentist - S7-E2
Character mistake: When the dentist is putting cotton in Jack's mouth he neglects to put on rubber gloves first as dentists are supposed to do before any dental work.
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).
The Not-So-Great Imposter - S5-E9
Revealing mistake: The kitchen of the husband with the cheating wife is the same kitchen used for Chrissy's parents house in Fresno at the very end of the next episode end of episode "Jack's Other Mother" (S05E10).
Revealing mistake: When Jack and Janet are in the entry looking around for burglars, their voices are over-amplified, indicating they are in a sound studio.
Jack in the Flower Shop - S2-E19
Revealing mistake: Once Jack arrives home from his first day, Janet follows shortly behind him with her arms full of groceries. She hands the bag to Chrissy but watch at the bottom of the bag - it starts to come apart and Chrissy grabs the bottom of it and holds it together. The end sight gag is that when Jack storms out, and Janet storms off, Chrissy then goes to go to the kitchen, and the groceries fall out of the bag at the bottom - but it's because then she's holding the bag then around the center.
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.
Grandma Jack - S8-E7
Character mistake: Jack says to Larry (upon finding out "Grandma" Tripper won the contest), "You want me to dress as a woman?" as if he never did that before and thought the whole idea was crazy - but earlier in the series he dressed as Chrissy in full drag so he's not new to the idea like he made it sound. Larry also dressed as a woman in an earlier episode as well (just for the record).
Suggested correction: It could have been a quick comeback line as a joke. The way Janet was even saying it sounded that way. Also, technically speaking, she did have a car in the past. The trio had bought Roper's Car for a very short time (from an earlier episode) before Roper bought it back from them. Janet never physically drove it but at that time, she was ⅓ owner of it so technically she had a car before.
Yes, I always thought it was a quick comeback.