Episode #1.1 - S1-E1
Continuity mistake: As the guests enter the house for the first time, Doctor Edward Armstrong in his white suit is amongst the first ones to go through the door. Philip Lombard is right behind him, and he walks past the door when the doctor reaches for his hat - but in the next shot, the doctor already has his hat in hand when Lombard hasn't gone past the door yet. (00:12:45)