Continuity mistake: When Lewbert is on the ground and everyone's in the lobby, you can see that Carly's shoe doesn't have a large wart before Lewbert screams, "Where's my wart!"
Visible crew/equipment: When Spencer is replacing Lewbert, you can see the studio lights reflecting off the bronze dome.
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