Just before the location changes to Monte Carlo, Greer Garson says a line about only being a working arrangement. Michael Wilding starts to say something as his lips move when viewed from behind. The view is then showing his face in which, after a pause, he responds.
(at around 39 mins) As Nigel and Jane converse, the boom mic shadow can be seen in the mirror on the far wall moving across the door as it closes.