My Desperate Valentine - S2-E16
Other mistake: At the end of the episode Brandon mentions he dropped Emily off with her parents and spoke to them. In season 4 when he goes to look for Emily at her parents' house in San Francisco he asked the woman who answered the door if she was Mrs. Valentine, as if they had never met.
Everybody's Talkin' `Bout It - S2-E21
Visible crew/equipment: While the parent meeting is taking place at the Walsh's home, Donna's mother stands up to express her opinions. The camera pans out to a wide shot where a microphone becomes visible at the top of the screen. (00:12:00)
Everybody's Talkin' `Bout It - S2-E21
Visible crew/equipment: When Kelly and her mother are having dinner with David and his father at the Japanese restaurant, a boom mic is visible above their heads. (00:18:00)
Continuity mistake: Brenda goes to see a counselor about the robbery. She asks what to do about the nightmares. The counselor asks about her happiest moment. Brenda starts talking about her horse she had as a child. She starts out saying his name was Sebastian but by the end of her story she says his name is Sylvester.
Continuity mistake: In this episode, Steve breaks into the school to improve his school grades in order to get into USC, because that is where his Dad went. But when the gang does go to college at CU, Steve's father is an alumni of that college not USC.
It's a Totally Happening Life - S3-E16
Visible crew/equipment: When Brenda and Dylan are in the same bedroom and the camera is panning about, you can see the end of the set in the upper left corner. (00:18:35)
Other mistake: When Dylan opens up his locker at school, he only turns his lock dial to the right. (00:06:00)
Visible crew/equipment: When Jackie and Babette talk with Cindy about their "good old times," a boom mic dips into the shot.
The Child Is Father to the Man - S3-E22
Visible crew/equipment: When Dylan calls Kelly at the end of the episode, in the shots of Dylan, a boom mic is visible above his head.
Continuity mistake: This is the first episode in which Kelly celebrates a birthday, and it is in March. A few seasons later, Kelly has another birthday, but this is celebrated in one of the first episodes of the season and takes place when everyone has just returned from their summer excursions. This would put her birthday in September.
Senior Poll - S3-E25
Visible crew/equipment: When Kelly returns home after her talk with Dylan, a boom mic is visible above her head. (00:37:45)
Continuity mistake: At the pre-prom party, the head in Kelly's glass of champagne disappears. No time has passed, so it hasn't gone flat. (00:25:45)
Other mistake: Throughout the entire 4th series and even various times into the 5th series, pay attention to the shots that show the outside of Steve's Kappa Epsilon Gamma frat house. Every single time, you will notice the same exact two people outside of the frat house - one is a guy wearing a green and white striped shirt sitting in the lawn reading a book, and the other guy is wearing a brown or orange shirt lying on the couch outside. While not impossible, these same exact two characters wearing the same exact clothes doing the same exact things outside of the frat house every single time for upwards of a year or more in 'show time' is unlikely.
Continuity mistake: During the scene in the kitchen where Brenda is at the table and her parents are talking to her about Stuart and then ask her if she wants a piece of pie, her arms are out in front of her on the table at this point. A split second later (same sentence), a rear camera view now has her left arm up holding her chin. A split second later after that when she answers "no", the front camera view now has her arms on the table as they had been all long.
Twenty Years Ago Today - S4-E8
Continuity mistake: In the scene where Brandon is going back to Dylan's house to retrieve his parents' wedding anniversary gift, when he is in the car and when he jumps out hurrying to the house he has his sunglasses on, but in the next shot with a different camera angle, as Brandon continues to hurry to the house, he has no sunglasses on. (00:22:00)
Continuity mistake: During the scene where David, Donna, and Andrea are in the casino and Andrea puts one dollar in a slot machine and hits the $500 jackpot, she screams and David tells Donna to get some buckets so that they may put all the coins in them. At that exact moment, they are interrupted by a security guard who asks them for their id's and when he figures out they aren't 21, he asks them to hand over the buckets of coins. At this moment, David and Donna hand over at least one bucket of coins. However, between the time David told Donna to get the buckets and when the security guard questioned them, the gang did not (and had no time to) get to fill up any of the buckets full of coins.
Radar Love - S4-E12
Audio problem: During the scene where Dylan and Kelly excuse themselves from the Walsh Thanksgiving dinner to go argue in the frontroom, pay attention to the camera shot behind Dylan when he is telling Kelly that he "just happens to like Cindy's Thanksgiving turkey better than your mom's." His lips do not match what he is saying.
Emily (a.k.a. The Pink Pearl) - S4-E13
Audio problem: During the scene where John Sears and some of the keg brothers are telling Steve that one of his hazing duties is to steal Professor Randall's signed baseball, pay attention to the camera shot behind John Sears when he is talking to Steve - his lips do not match what he is saying.
Somewhere in the World It's Christmas - S4-E15
Revealing mistake: At the end of the episode in the Peach Pit, Nat and the gang are all singing Christmas songs while Nat is playing some of the songs on a keyboard. It is very easy to tell by looking at his hands, Nat is simply miming on the keyboard.
Somewhere in the World It's Christmas - S4-E15
Revealing mistake: Pay close attention to some of the scenes on the airplane during the flight that the Walshes are taking to Hawaii. Several times you will see a large, winding (and ascending) staircase in the background of the plane. If you also look close, you will see an 'exit' sign' slightly behind and to the left of the staircase. While some luxury airbuses do have staircases, given the fact you can see an exit sign it is easy to tell these scenes were shot in some kind of setting other than an airplane.