Revealing mistake: When Basil, Manuel and Polly are trying to hide Mr. Leeman from the other guests, they put him in a cupboard. While they are talking to the people staying in the room, Mr. Leeman's hand comes out of the cupboard. Watch carefully, and you can see the actor opening the door from the inside, and then putting his arm down, making him look like the corpse.
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?
Visible crew/equipment: When Basil, Manuel and Sybil enter the office and find Miss Tibbs fainted next to the corpse, the boom mic appears on the righthand side of the screen next to the fan on the wall. And when the camera switches back to the full shot of all the characters, the boom mic appears again.
Revealing mistake: When Basil sneaks into Raylene's room to listen for the "extra guest" in Johnson's room, Raylene wakes up screaming. When Sybil comes in, turns the light on and Basil says "I didn't know she was there, I just came in to check the walls", look at the extreme right of the screen. You can see the shadow of a boom light on the wall.