Visible crew/equipment: When Basil's trying to persuade Sybil about the woman in the guest's room, the shadow of a boom mic appears on the top wall.
Visible crew/equipment: After Mr. Johnson refers to the Torquay guide as the 'world's shortest book' and leaves the hotel, you see a boom mic coming in at the top of the screen for about 3 seconds.
Revealing mistake: When Basil is climbing up the ladder to view the inside of Mr Johnson's room, he accidently peers inside Dr Abbott's room. When he starts doing his "checking" routine on the window before he falls backwards, the pane flexes, indicating that it's made of clear perspex.
Revealing mistake: There are noticable disparaties between the locations of rooms and the windows on the outside of the building. This is particularly noticable in this episode when Basil climbs a ladder to look through the window.
Revealing mistake: When Basil sneaks into Raylene's room to listen for the "extra guest" in Johnson's room, Raylene wakes up screaming. When Sybil comes in, turns the light on and Basil says "I didn't know she was there, I just came in to check the walls", look at the extreme right of the screen. You can see the shadow of a boom light on the wall.
Continuity mistake: In this episode, Basil hides in a closet to try and trap a girl staying the night in a guest's room. However, in other episodes ("Wedding Guests," "Communication Problems," and "The Kipper and the Corpse"), there is a room in the same spot the closet is in this episode.
Continuity mistake: After the woman screams, Sybil knocks the wall and you can see it move.
Continuity mistake: In the episode "The Wedding Party", the first two rooms on the second floor right of the stairs were 11 and 12. Yet in this episode, those two exact same rooms are now 5 and 6. (00:19:20)
Visible crew/equipment: Basil runs out of the kitchen and stops Doctor Abbott and starts talking to him. The shadow of a boom mic appears on the wall to the right of the screen twice as he talks to him. (00:15:25)
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.