Continuity mistake: Near the start when Moss runs and jumps onto the couch, his tie changes positions repeatedly between shots. In close-ups it is up on the couch arm and in all other shots it is under Moss.
Continuity mistake: Season 2, episode 5 "Smoke and Mirrors": Roy's lipstick alternates between bright pink and more of a peachier colour throughout the scenes.
Continuity mistake: Season 3 Episode 1 'From Hell' When Roy says "Nice glasses" and Moss replies with "Not as nice as your momma's glasses", The Red Door between Roy changes from being closed to being open.
Continuity mistake: The way Jen's books are stacked changes after she says "pedal stool." The book with the red pages moves to the top.
Answer: It's a joke on the "118" numbers in the UK. 118 numbers were used for directory enquiries and adverts for them on TV always tried to give a way to easily remember one for a particular company. The fake ad in the show takes it to the extreme. People in the UK do not use "911", the point of the joke was that Roy had forgotten "999" while saying it was easy to remember.