After talking to Sybil in the kitchen, Basil runs through the dining room and into the lobby to speak to the doctors. Immediately thereafter he checks in an attractive guest. Sybil emerges from the back offices, never crossing the lobby to get there.
When Basil taps on the window of the Abbotts' room, the "glass" clearly flexes under the pressure.
A shadow of the camera can be seen during the scene where Basil loses his temper and tries explain to Sybil that there is a girl in Johnson's room.
A boom mic is clearly visible in the top right-hand corner of the screen for several seconds (until the camera moves down to compensate) when Basil is talking about swimming to France.
When Sybil says "sledgehammer wit," the mic above her dips down into the picture, above Mr. Johnson's head. The camera then sneaks down the shot to help avoid it.
There's a shadow of a boom mic on the wall as the lights come on when Basil's trying to listen at the walls in a young woman's room.
The shadow of the boom mic is visible on the hallway wall in the top right next to Johnson's room just before Basil and Sybil confront him in the final scene.
When Mr. Johnson picks up the phone at the front desk, he accidentally rings the bell with the side of his hand. (However, Prunella Scales quickly stops the bell from ringing and continues with her next line as if it's no big deal.)