Continuity mistake: When Sam bangs on the locker with the hammer, the amount of damage on the metal repeatedly changes; it decreases, increases, decreases, and so on.
Revealing mistake: When Carly's in her dream and attempts to use the elevator, if you look really closely, she doesn't actually hit the button that lights up.
Answer: In direction translation it means 'Mischievous Spirit'. 'Hob' as in HobGoblin means clown or prankster, and 'Knocker' was an old age term for Poltergeists in mines. Miners would often complain of hearing knocking sounds down mines when there was no-one else down there, hence 'Knocker'. It's not a regular English expression though.
GalahadFairlight