Continuity mistake: When Rachel and Finn are doing their toast, the amount of cider in the glasses changes between shots.
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.
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: 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)
Continuity mistake: During "Control", Blaine's jacket can be seen repeatedly switching from closed to open. Most noticeably when the chorus shouts "oh yeah" and he's seen high-fiving another member of the group on the right of the screen. This is shown two consecutive times from two slightly different angles. The First Time his jacket is open, the second time it got buttoned up. (00:28:25)
Continuity mistake: When the Warblers fill the cup up with rock ice, you can see the cup is empty, yet in the next shot it's full of Slushie and no rock ice is in there when it's thrown in Blaine's face.
Continuity mistake: Just as Kurt is pulling up a chair next to Blaine in the Lima Bean scene, Blaine's arm is down by his side. Yet when Kurt hooks their arms together in the next shot, his arm is on the table.
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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