Continuity mistake: Near the beginning, when Sybil goes into the office after speaking to Basil about the date, the lights are obviously on in the office, but when she walks out, it can just be seen on the right that the office is in complete darkness, and then when the shot changes back to the regular one in the lobby, the office is lit up again, without anyone going in there in the meantime.
Plot hole: Sybil leaves the hotel at 05:29 and returns with Audrey at 24:39. 19 minutes have elapsed in real time during which Basil has been 'entertaining' Sybil's friends. So Sybil had just enough time to visit Audrey, talk to her and return. There wasn't enough time to visit the homes of Alice, Kitty or Virginia. Sybil went straight to Audrey who must therefore be her closest friend. Also Audrey was free and knew nothing about the party. Would not Basil have invited Audrey to the party? Is not Audrey's presence just to allow Sybil to explain to the audience when it makes no sense she is not at the party?