Continuity mistake: Puck butters the floor, and as Kurt slips and falls, The Substitute is not where she had been standing. The next shot, she's still there. (00:07:45)
Continuity mistake: During "Blame It on the Alcohol", when Quinn is shouting at Puck, her hair is messy and straight. Yet a little while after, during "Don't You Want Me", her hair is curly and styled perfectly.
Continuity mistake: During "Last Name", just as April is singing "We left the club," she backs up into Kurt in the wide shot, yet during the very next shot she is still a couple of feet away from him.
Sectionals - S1-E13
Continuity mistake: In episode 13 of season 1 (Sectionals), during the five way split screen, the extras behind Mercedes and the ones behind Tina are the same but have different movements.
Continuity mistake: While the Warblers are singing "Teenage Dream", during the closeups of Kurt, his bag's strap is under his jacket's lapel, but in the long shots, it's on top of the lapel.
Continuity mistake: When Kurt goes to sit down and play the piano for Finn, he is wearing cream coloured pants. Yet in the closeup shot of his hands, he is wearing jeans.
Continuity mistake: In 3x09, while Kurt and Blaine are singing "Let It Snow", the fire is going. When they finish the song and notice the "audience", it's out. When Kurt is opening his present from Rachel, it's going again.
The Purple Piano Project - S3-E1
Continuity mistake: Blaine has finished singing and the piano is ablaze. Several Club members are seen sitting at the table, then a different view shows only Rachel and Kurt and empty seat spaces where others were sitting.
Continuity mistake: When singing in the choir room with Finn, Rachel's necklace changes from inside her dress to outside several times.
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.
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.
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)
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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