Patzer

In an episode following the death of Noah's father, Noah is showing Valerie his parents' home. While touring the house, Valerie enters the kitchen wearing a different pair of shoes than in the previous room.

In the first season, Donna Martin's mother was named Nancy Martin, and played by Jordana Capra. When she was reintroduced in season 2, she was named Felice Martin, and played by Katherine Cannon.

Steve has shaving cream on his ears, but when the camera goes to a different angle the shaving cream is gone, and again when it goes back to the other camera angle, the shaving cream appears again.

The exterior of Dylan's house changes without him moving. The first outside shots show a white house in a comfortable neighborhood. Later outside shots show a dark wood house set back from the street and down a slight incline.

In the first season, when Donna tries out for school DJ, she is referred to as Donna Morgan. Throughout the entire show, her name is Donna Martin.

Throughout the series, it's clear that the actors/actresses portraying teenagers are actually in their 20s+. This is done to get around child labor laws, that won't allow teenagers to work full time on a set due to interfering with their real-life schooling. Plus casting adults adds to the benefit of better acting from the actors/actresses due to having more experience plus the producers are able to sexualize the characters without consequences. Lastly, it gives the general viewing audience who are teenagers (or young people) someone they can look up to.

In the pilot, Brenda tells her date she's a junior at West Beverly, which means she and the others would be due to graduate at the end of the second season. But they don't graduate until the end of the third season.