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: As far as the "gliding in and out", Mrs Loren presumably uses the same body harness she used to fake her hanging. As to the rope... the film is deliberately ambiguous on whether or not the ghosts are real, as evinced by the drunk's last line in the movie; the rope trick is probably supposed to reflect this ambiguity despite being Mrs Loren's trickery.