Continuity mistake: There appears to be a very obvious splice edit in this episode. When Basil manages to get Kurt off him after being pinned to the kitchen table, he pushes Kurt against the wall knocking him out. Before this Basil is lying flat on the table and we never see him get up to a standing position beforehand.
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.
Continuity mistake: Towards the end of the episode, a couple of scenes before Mr Stubbs arrives, we see Basil start up the tape player prior to hiding in the kitchen. The garden gnome on the desk is right next to the tape player at this point. When Sybil turns the tape off the player has rotated 45 degrees or so (despite nobody else having come into reception in the intervening time) and the gnome is now further along the desk towards the camera as we look on the wide shot. The gnome in the next shot is seen beside the checking-in book when Basil wanders out of and back into the office a couple of scenes later, yet when Mr Stubbs is asking about the lintel for the new door shortly after that and we see Sybil writing in the book, the gnome has moved out of shot altogether.
Revealing mistake: When Basil is out of the car and shouting at it after its final breakdown, you can see a crew member reflected in the right hand side of the windscreen giving John Cleese cues. It's not Cleese himself, as the reflection remains after he runs out of shot to get the tree branch, and the reflection doesn't mimic every action performed on screen.
Visible crew/equipment: Near the start of the episode, after the guest has told Basil that he will only be staying until Sunday, Basil walks from the dining room to the office, and the shadow of the camera following Basil can be seen in the reflection of the studio lights in the picture above the Drawing Room door.