Plot hole: Even if Dr. David Trent's and Annabelle Loren's elaborate plan had worked flawlessly (framing Nora Manning for Frederick Loren's death), the fact remains that they faked Annabelle's death for everyone to see, which would immediately arouse suspicions for investigators. Even if Annabelle and Dr. Trent somehow fled the scene before the police arrived the next morning, their actions would still raise many questions that implicated them; thus, their clumsy and convoluted scheme was far from being the "perfect crime" they imagined.
Plot hole: When Mr. Loren introduces his wife to everyone, he tells her, "You know Watson Pritchard of course." That can't be because at the beginning of the movie the characters are asking if any of them know Frederick Loren. No one personally knows him including Watson Pritchard. He says, "I don't know him, all the details about renting the house were done by mail." If that's the case, it's impossible for Annabelle to know Pritchard since Frederick invited all the guests without his wife's approval. (00:34:05)
Chosen answer: Frederick knew that Trent was his wife's lover. That's why Trent was invited. The party was supposed to be an opportunity for Frederick to take revenge.
Charles Austin Miller