Audio problem: In "Silly Love Songs" during the final song You can hear Blaine singing "I can explain." but in a couple of the shots he is lip-syncing to the "I love you". Next shot is he is back to singing his solo part.
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.
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.
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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