Other mistake: When the show aired in 1990, in the pilot episode Brenda falls for Jason. She later on reveals herself to be a "16 year old junior" in high school. Season 2 opens up with the gang on summer break which technically would make them going into their senior year. It wasn't until Season 3 that they were going into their senior year. They graduated in 1993 from West Beverly but the show debuted in 1990 so technically they should have been sophomores.
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.
Other mistake: When Dylan opens up his locker at school, he only turns his lock dial to the right. (00:06:00)
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.
Other mistake: There was an error regarding the captions on this episode. When "I'm Gonna Get You" was playing at the rave, the captions displayed the lyrics for "Every Day of the Week." Then when "Every Day of the Week" was playing, the captions displayed the lyrics for "I'm Gonna Get You."
Angels We Have Heard on High - S6-E15
Other mistake: In the scene at the Peach Pit where Rush tells Steve that he is his biological father, he shows Steve a copy of the birth certificate. Under adoptive parents, it lists Sanders, Rush and Sanders, Iris. Only Iris isn't Steve's adoptive mother, Samantha is. Iris is Dylan's mother who lives in Hawaii.
Other mistake: In this episode, Felice and Dr. Martin are supposedly celebrating their 25th wedding anniversary. In the sixth season episode "Earthquake Weather, " Felice told Donna that she had to drop out of the Rose Court competition after getting pregnant. She got married at that time, and suffered a miscarriage. In the earlier episode "Offensive Interference, " Donna saw her mother in a photo of the 1969 Rose Court contestants. Therefore, Felice and John should be celebrating their 29th anniversary, not 25th.