Visible crew/equipment: Soon after Dennis dies via the Hobgoblin induced fantasy, Mr. McCreedy comes to check on him; when he gets to the door where they stopped earlier (where McCreedy told him not to proceed further down the hall), the actors' marks, T-marks in black duct tape, are visible on he floor where they stopped earlier.
Factual error: The damage done by the hobgoblin-induced fantasies is real, as seen several times in the movie. Kyle's car runs off a cliff, Nick's grenades really explode, etc. Nick is in one scene set ablaze by throwing himself on a grenade, and is completely engulfed in flames from his head to his toes. But he shows up in the last scene with nothing but a few minor burns on his forearms and a pair of crutches (that he immediately discards). His hair and eyelashes are not even singed.
Plot hole: After Dennis (the first guard assistant) gets killed, the vault is open for several minutes before the old guard arrives. The hobgoblins, however, do not run for their freedom, but patiently wait until the hero, Kevin, comes in a later scene and opens the door. Then they escape as quickly as they can.
Continuity mistake: Many of Daphne's accessories change throughout the film. For example the small blue badge she wears on her shirt randomly changes its whereabouts on the shirt, jumping about from scene to scene. When Daphne and her friends arrive to Club Scum, Daphne has none of her many necklaces on. When they enter the club, she now does, and throughout the Club Scum scene, they disappear and reappear as well. When the hobgoblins come into the living room, where the music is playing, they attack Daphne, Amy and Kyle, and Daphne's sunglasses alternate from sitting on her nose, to up on her head throughout this scene, and in following scenes too. Finally, Daphne has a pair of sunglasses which hang from the top of her skirt, and between shots (especially the Club Scum scene) the sunglasses change from hanging on her left side, to her right. The only hilarious highlights in an utterly pathetic movie!
Plot hole: The hobgoblins are locked in a security vault to ensure they never escape and kill people. In addition to the heavy vault door, there are two steel bar doors and an additional door up the corridor protecting the vault. None of these doors are ever locked, even after the first security guard walks in and gets killed. Even the burglar walks straight through these doors, so that Kevin has to follow him and let the hobgoblins out.