When Adam is trying to persuade Margaret not to go to Hendrick's house, he gives her a drink in a sherry glass. At the end of the scene, she gives him back the empty glass - which has suddenly become a whiskey tumbler.
Margaret and Sandra arrive at Hendrick's house for dinner at 7.30pm - but it is quite dark outside when they arrive, even though it is June and should be light for a couple of hours longer.