In the beginning credits of the movie, it states, "Radio music supplied by Charles Greene and Brian Stone". The music that they used was "Green Rocky Road", performed by the band "The Daily Flash", which Greene and Stone managed.
According to Peter Bogdanovich's introduction to the film on the DVD, the scene in which Sammy wakes up with a hangover and is jolted when the first thing he sees is Byron Orlok originally called for him to laugh at himself, but he couldn't manage it. Boris Karloff suggested the bit that is actually used in the film - and ad-libbed Orlok's start when he unexpectedly confronts himself in a mirror, as well.
The mother's body temporarily disappears from entryway to room where she was shot when Bobby carries his wife's body into another room, but somehow reappears when he returns.
Bobby goes to the gun shop to stock up on ammo. While there he's eating a Baby Ruth candy bar. The candy bar stays its full length between shots even though Bobby is obviously consuming it. This indicates several takes of the same shot.
(at around 46 mins) After Bobby took his dead wife very close in his arms, his hand is full of blood from her back, but his shirt stays clean, even with the large blood stain she has in front.
Byron Orlok: [as he being driven to the drive-in theater, looking out the car window, while passing many car dealerships]
Gosh, what an ugly town this has become.