Continuity mistake: Frederick picks up a bottle of champagne out of the ice bucket and shakes it a little. As he does this the foil and wire are still on top. He turns and aims the bottle at his wife, then tips it back and the foil has vanished as he now just removes the wire and cork.
Visible crew/equipment: When the severed head in Nora's suitcase has vanished, she shouts for everyone to get out and leave her alone. In the hallway when they disperse, as Loren and then Trent walk past Lance, something that shouldn't be there (it looks like fingers perhaps) pops up into view at the bottom, left corner of the screen. This is clearer in the colorized version of the film. (00:49:00)
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