Continuity mistake: When singing in the choir room with Finn, Rachel's necklace changes from inside her dress to outside several times.
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.
Answer: According to IMDb.com, Ian Brennan, one of the "Glee" writers and producers, is listed as the uncredited narrator voice.
Michael Albert