Factual error: In season two the show makes Dalton look like it's in a suburb of Lima. Dalton Academy is suppose to be located in Westerville, OH, which is two hours southeast of Lima. Kurt & Blaine would not be able to go to the "Lima Bean" coffee shop every day if this was the case.
Continuity mistake: During Season 2, Episode 9 (Special Education), just as Blaine is singing "Hey soul sister, I don't wanna miss." for the last time, he is several feet in front of the other Warblers. But during the next shot, as he is singing ".a single thing you do.", he's right in front of them. So he jumped back several feet between two shots.
Continuity mistake: During "Theatricality", when the boys are singing "Beth", just as Puck sings "Me and the boys are Playin'" for The First Time, Finn is visible sitting behind Quinn. Yet Finn went to sit down and sing with Puck and is obviously sitting with the boys in the next wide shot.
Continuity mistake: In "Hell-O", as Santana and Brittney are talking about the "hottest guys in the school", Santana's glass is almost empty. Yet two shots later, when she takes a drink, her glass is full. Also, when she says "Would you mind waiting in the car?" her glass is full and you can obviously see a slice of lemon in it. Yet in the next shot, when she says "And leave your credit card", her glass is only about 1/3 full and the lemon slice is gone.
Other mistake: When the kids are opening their lockers, they enter the code, then pull on the lock to open the locker. The closed lock is still attached to the front of the locker. When they close it, it's magically locked again without them ever opening or closing the lock. In reality, these locks would be useless, or the locker doors wouldn't open.
Other mistake: Many of the Warblers appear in McKinley throughout the show, even after the Warblers were introduced. An example of this is that Trent is often seen walking throughout the halls of McKinley. He even sits in front of Finn in "Substitute" when Holly Holiday is teaching the students Spanish.
Continuity mistake: Kurt informs Blaine that he flew in an airplane for The First Time. This however is incorrect, as Kurt flew to Albuquerque NM for the 2010 Cheerios Championships. In the season 1 episode Sectionals it is mentioned the Cheerios Nationals are in Albuquerque and in the season 1 episode Funk The Cheerios return from Albuquerque the same day. 1500 miles in less than a day means they flew.
Other mistake: When Finn and Rachel sing Smile in the Choir Room in Mattress (Season 1, Episode 12), Finn doesn't appear to look at any other page but the first one in the music given to him by Rachel. It is unlikely that the entire score for the song was on the first page.