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).
Helen's Job - S2-E13
Visible crew/equipment: Janet stops Helen from leaving, says she can't handle seeing her beat. After Helen's speech about winning the war, as the camera zooms out, a piece of set equipment is briefly visible at the top of the screen. (00:16:55)
Ground Rules - S2-E1
Visible crew/equipment: Shadows of the boom and a crew person moving reflect off Jack's hair as he's talking with Janet in the kitchen.
Visible crew/equipment: A crew person's arm or equipment can be seen at the far right of the screen, when Mr Roper asks Jack for some baked beans, after the opening credits. (00:01:10)
Revealing mistake: The lights around the stage reflect in Jim Walsh's sunglasses a few times.
Continuity mistake: In episode 8-3, they got a new couch. In this episode, the old couch is back. (00:05:01)
Suggested correction: Many episodes in Season 8 were aired out of sequence from how they were actually filmed. The production code for this episode was 801, and the code for the episode where they got their new couch was 804 (made 3 episodes later - even though it was actually aired first).
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.
Revealing mistake: Studio lighting on Janet and Chrissy's tent at night, in the middle of the woods. (00:21:12)
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)
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.
The Case of the Missing Blonde - S5-E21
Plot hole: After being stuck inside the phone booth, the police officer arrests Larry for vandalism after Larry breaks the glass in the phone booth. In a real-world situation, an officer would most likely let someone go without arrest if the person got stuck inside like that (declaring an emergency) and couldn't get out without breaking the glass.
Helen's Job - S2-E13
Factual error: After Mr. Roper stirs Jack's batter, Jack pours it into the cake pan and then opens the oven, leaving plenty of batter residue inside the mixing bowl, which a professional chef student would never do.
Factual error: Jack takes Cheryl's phone off the hook and puts it down, but while the two are talking throughout the two-minute scene, the phone does not make the loud annoying buzzes that occur after 30 seconds or so from the phone being off the hook (which happened back in the years the show aired).
