Boss Man refers to the twitching/sudden movement of one of the corpses as a "death rattle". A death rattle is a rattling sound produced in the throat as a person dies, and has nothing to do with the physical movement of a corpse.
The point in the movie where beer-guy (Judah Friedlander) is getting washed after being 'slimed' (22:10 to 22:35), the amount of maggots covering him (especially his hair) changes drastically between shots.
The payphone gets shot off the wall and the guy goes to hang up the receiver but, obviously, it falls as there is no longer a payphone there but a hole. In the next scene the receiver is hung up in the wall.
Near the end of the film when the monster has crashed through the wall of the bar, the monster rips off his deer skull mask/helmet to show his true face. As soon as he tears the mask/helmet away, look carefully to the right of the screen. For a split second, you can see the black outline of a person standing beside it. This is revealed in the commentary.