Visible crew/equipment: Near the end, 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.)
Plot hole: If the creatures are strong enough to simply burst through the walls, as shown at the end of the film, then the creature upstairs should not have been stopped by the small door that Bozo, the bartender and heroine shut on it to stop it. It was one of the larger creatures as well, so it could have easily gotten through.