The opening credits say the music is based on Mussorgsky's Pictures OF an Exhibition. The actual title of Mussorgsky's piece is Pictures AT an Exhibition.
When Mary looks at Paul's watch on his right wrist as Big Ben strikes midnight, he is wearing the watch upside down from his prospective - but so the viewer can see it's midnight as the watch is turned to face the camera.
In the brief scene where Paul Mitchell goes to an art gallery looking for a work by Henry Mason, the shadow of the boom microphone can be seen moving back and forth on the upper wall behind Mitchell and the gallery's proprietor.