Deliberate mistake: During almost every scene that involves the main actors in the hallway or in a classroom with a window to the hallway, there are people passing by at random intervals. Even if the scene takes place in the middle of class or after school, there are random extras wandering the halls. In real life, unless they had about 15 minutes between each class and every single club met after school every single day, there wouldn't be that many people in the halls.
Visible crew/equipment: In "The Power of Madonna", just before Sue drops the Madonna CD onto Figgin's desk, a camera is briefly visible over her right shoulder.
Britney 2.0 - S4-E2
Continuity mistake: In Britney 2.0, when Brittany and Santana are video calling, look at Britt's laptop. Don't look at Santana, look at Brittany. In the first shot, there's some kind of light probably a window, at the back of Brittany. In the second shot, there's no light or window at all. (00:04:10)
Extraordinary Merry Christmas - S3-E9
Continuity mistake: When Kurt and Blaine are singing "Let It Snow", during their last long note, Kurt places his hand on Blaine's back. In the next shot, it's back by his side.
Continuity mistake: When Artie tries to walk with crutches after the "step dance" practice with Tina, he falls down violently. Immediately after the fall, the crutch is still attached to his left arm, which is angled away from his body. In the very next shot, the crutch is almost a foot away from his arm, which lies now at a different angle. (00:10:05)
Continuity mistake: In 3x09, during the opening number, Kurt is wearing the gum wrapper ring he received as a gift from Blaine. Although the scene where he receives it was cut from the aired episode, he wasn't supposed to receive the ring until later in the episode.
Factual error: After he gets fired from his teaching job, Mr. Ryerson makes a living reselling the medical marijuana he receives for depression. At the time the episode was made, Ohio was not one of the 16 states (and District of Columbia) that had medical marijuana laws, and NONE of the states allowed it for treatment of depression.
Continuity mistake: When Sue and her mother are singing "Ohio", during the closeups, Sue has her arm around her mother's waist, yet in the wide shots, Sue's arm is hanging by her side.
Continuity mistake: During "Grilled Cheesus", the grilled cheese sandwich with the face of Jesus changes occasionally between shots. Sometimes the burn is slightly darker, or the edge is changed.
Continuity mistake: During "Home", when Mercedes is singing "Beautiful", Quinn stands up to join her. When she passes behind Mercedes, she stops and puts her hands on her stomach. Yet in the next shot, she passes Mercedes a second time and once again stops with her hands on her stomach.
Audio problem: During "Marry You", in the wide shots where Carole is coming down the aisle and the Glee kids are in the background, the lyrics and their mouth movements don't match up.
Continuity mistake: Near the end of the episode, when Mr Schue is talking to Terri in the school hallway, there are several people walking around in the background and there is even one student running towards the camera. In the next shot - one second later - the hallway is completely empty.
Continuity mistake: During Mike's "Bubble Toes" dance number, the chain on his pocket watch switches between being over a foot long, swinging about as he dances, and being only a few inches long. (00:25:40)
Audio problem: During the beginning of Rachel's "Baby One More Time" fantasy, the tapping sounds and her pencil's movements don't match up.
Prom Queen - S2-E20
Continuity mistake: When Blaine and Kurt are in Breadstix talking about Prom, the amount of soda in both of their glasses changes between shots.
Continuity mistake: During "Bad Reputation", as the kids are singing "Ice Ice Baby", their sheets of music continually change position, especially with the people sitting down. For example, in the first shot Mercedes is holding her music in her right hand. In the next shot it's down on her lap. In the third shot she's holding it with both hands, and in the last shot it's in her right hand again.
Continuity mistake: When Lauren drags Puck into the storage closet for Seven Minutes In Heaven, there are hearts on the wall which appear to be decorations for Valentine's Day. As the Seven Minutes In Heaven happened prior to Sectionals, which was well before Valentine's Day, these shouldn't be there.
Continuity mistake: When Will reads the newspaper review of their concert, the date on the newspaper is February 12, 2009. This date is impossible. Most of the Glee kids who graduated in 2012 were sophomores in season 1, and the only way to be in school around the February 2009 date is if the class of 2012 were 9th graders. Only this is not the case. Also, all the football or basketball episodes in season 1 would all be incorrect. The most likely date for Acafellas is September 2009.
Continuity mistake: The DVD Tina brings Mr. Chang. When she leaves she picks it up from the desk, in the last shot it is back on the desk. (00:17:38)
Answer: The show is purposefully stylized and over the top. It is not designed to be an accurate, realistic representation of American high school. The idea that the glee club is so unpopular other students can literally throw garbage at them with impunity is an over-exaggeration of how glee club is considered "un-cool" compared to things like football. However, there is a second way of looking at it. The audience is simply never shown anyone getting into any trouble for throwing slushies. Just because we don't see it doesn't mean it never happened.
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