Other mistake: When Mason is punching the air and Nina imitates him the chair wiggles with the motion of her body but since she is dead she would pass through it.
Continuity mistake: When George writes Nina's name on the sticky note she holds it up and we see the writing is small but then we see a close up and the writing it bigger and the handwriting is a little different.
Chosen answer: The Song is Callled "Boom Boom Ba" and is by the band Metisse. it plays at certain points during the series. Mainly when George is looking over Reggie or in Reaping Havoc where it plays whilst shes showing her scrapbook "Mysterious and Reassuring" which is what she thought Betty was.