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.
Deliberate mistake: During almost every scene that involves the main actors in the hallway or in a classroom with a window to the hallway, there are people passing by at random intervals. Even if the scene takes place in the middle of class or after school, there are random extras wandering the halls. In real life, unless they had about 15 minutes between each class and every single club met after school every single day, there wouldn't be that many people in the halls.
Continuity mistake: In 3x14 (On My Way), Quinn apparently texted Rachel that she'd be there soon at 11:01PM on February 9, 2012. Quinn got Rachel's text back at 4:14PM. When we see Rachel's phone again, her text saying "HURRY!" is gone. (In addition, Feb. 9th 2012 was a Thursday, not a Saturday).
Factual error: After he gets fired from his teaching job, Mr. Ryerson makes a living reselling the medical marijuana he receives for depression. At the time the episode was made, Ohio was not one of the 16 states (and District of Columbia) that had medical marijuana laws, and NONE of the states allowed it for treatment of depression.Captain Defenestrator