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: 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: Puckerman is on a skateboard and then jumps on to the bench, but the skateboard just disappears.
Prom Queen - S2-E20
Continuity mistake: As Kurt and Dave are talking in front of Kurt's French class, a short girl with a brown ponytail enters the class twice.
Continuity mistake: In the "Singing In the Rain" dance, the second time the three guys are on their knees, they splash water with their hands. The third time they splash, the water drops are moving toward the stage instead of away. And the rain in the background is falling up.
Deliberate mistake: When Howard enters Figgins' password to hack into his AOL account, Figgins' inbox is obviously visible behind the login box, which simply disappears once the password has been entered. In addition, look closely at the inbox and you'll see that not only is there no mail, they are actually in GMail (the logo has simply been edited out of the top corner).
Continuity mistake: While Santana is singing "Back to Black", Jessie is seen obviously scribbling something down. Yet at the end, his notepad is shown with a drawing of a cat upon it. If he was drawing the cat, he would have been drawing smooth lines, not scribbles, as the drawing contains no scribbles at all.
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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