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.
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.