Other mistake: When Rube is searching the hard-copy (paper) archives, the young man with him opens the last box and pulls out an old "Wanted" poster with Rube's face on it. He tells Rube "This guy looks just like you." Since season 1, episode 1, we've known that Grim Reapers don't look like they did when they were alive (i.e., Millie/Un-George). Yet somehow, Rube has not changed since his life/death.
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.
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.