Revealing mistake: During Jack's dance scene, the old lady that he dances with spins away from him and knocks off a man's toupee. The actress was supposed to knock it off as she went by, but it's obvious that she misses and instead quickly reaches over to snatch it off his head instead.
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.
Continuity mistake: After Furley tells Jack about picking up the pot, he says, "Of course, it'll cost you." Later, when the cop listens to the recording of the conversation, the tape has Furley saying, "Of course, it's gonna cost you."
The Not-So-Great Imposter - S5-E9
Revealing mistake: In Angelino's kitchen, when Mr. Angelino comes in and mentions steaks for eight people are needed, look behind him at the door's window. Janet is briefly seen watching (as if waiting for her cue to enter), then quickly ducks out of sight. A second later, she's seen again while Angelino asks Felipe to help him in the freezer. She then runs in the door with Furley as if they first arrived at that moment.
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.
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.
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.
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?
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.
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).
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.