Plot hole: Noah cancels the club's scheduled Monica concert because Dylan pays him to have her give a private performance to Dylan and Gina instead. He seems really excited and proud of himself for some reason. Even if Dylan paid him much more than he would have received from the gate (which is likely), this could cost Noah money in the long run. Many customers would probably get angry and decide to stop frequenting a place that would cancel a concert at the last minute for no good reason. Then again, Noah was never exactly a genius - in business or otherwise.
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.
Character mistake: When he arrives at his hotel room in Vegas, David is surprised that Ray is performing at one of the casinos. But at the beginning of the episode, Steve told Brandon that Ray was performing, which suggests going to see him was part of their plans. So either (1) Steve shouldn't have known Ray was performing there, or (2) David shouldn't have been so surprised.Cubs Fan
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.
Continuity mistake: In the beginning of the episode Brenda takes her mom's coat to drama class and is wearing a blue shirt and white biker shorts. That night the parents go to the beach club. The next morning before school they show Brenda holding the coat and again in the blue shirt and white shorts she wore the "day before."CGuerra404
Visible crew/equipment: Kelly, Steve, Andrea, Brandon, and Dylan are all gathered near the lockers when Kelly begins to discuss her opinion of Steve's new girlfriend Christine. Just after Steve says "Tell me everything that's wrong with Christine", the camera pans to Kelly where a microphone becomes visible at the top of the screen. (00:15:15)Bk04
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.
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.