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: Many times throughout the show, a large bandage or makeup blot is visible on Lea Michele's right wrist, which is used to cover up her "I Believe" tattoo. This is most visible during "Don't Rain On My Parade" and her "Poker Face" and "For Good" duets.
Other mistake: During "Asian F", when Kurt is filling out his NYADA application, he lists his address as "415 Whitman Avenue". Yet during "The First Time", when Rachel is approaching Finn and Kurt's house, the number on the house is 291.
Continuity mistake: In 3x09, while Kurt and Blaine are singing "Let It Snow", the fire is going. When they finish the song and notice the "audience", it's out. When Kurt is opening his present from Rachel, it's going again.
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 3x09, during the opening number, Kurt is wearing the gum wrapper ring he received as a gift from Blaine. Although the scene where he receives it was cut from the aired episode, he wasn't supposed to receive the ring until later in the episode.
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.
Answer: According to IMDb.com, Ian Brennan, one of the "Glee" writers and producers, is listed as the uncredited narrator voice.
Michael Albert