Continuity mistake: Puckerman is on a skateboard and then jumps on to the bench, but the skateboard just disappears.
Continuity mistake: While Jake is singing to Marley in history class, the clock drastically changes times from one shot to the next.
Continuity mistake: When Sam is talking to and kissing Nurse Penny in her office, Nurse Penny's lanyard keeps moving from the left side (on screen) of her name badge to the right side and also sits on top of the badge. (00:43:00 - 00:45:00)
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)
Laryngitis - S1-E18
Continuity mistake: During the "Lady is a Tramp" number, Puck puts on a hat with the front of the brim flipped down. The brim miraculously flips up and back down throughout the scene.
Continuity mistake: In "Gives You Hell", while Rachel is sliding down the middle in the chair you can see Puck on the left then on the right. He couldn't move that fast in a second.
Continuity mistake: During the song express yourself, whilst the girls are dancing on the stage Quinn appears to have a flat stomach but she is supposed to be pregnant at this stage.
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.
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.
Factual error: They sing Katy Perry's "Roar" before the song was even released. This episode is set in Spring 2013 and the song was released in August.
What the World Needs Now - S6-E6
Continuity mistake: When Rachel is singing the final long note of "Promises, Promises," she slowly raises her arms to above shoulder level. Then, a shot from behind shows her arms well below shoulder level, then a return to a front shot shows her arms slightly above shoulder level again.
Other mistake: In the scene at the lockers with Brittany, Santana mentions having trouble finding a younger man per Sue's orders. However, Santana is a senior in season 3, which would make her a sophomore in season 1. Therefore, she could easily just date a freshman instead of needing Finn.
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.
BaconIsMyBFF