Plot hole: The group poison a dozen or so people and dispose of their bodies by burying them in their own garden. Some of their victims are notable celebrities - television presenters, authors, leaders of large and vocal pressure groups. Certainly some of them are nobodies, but nonetheless they are going to tell friends where they are going for dinner! Not one of these people told their assistants or their secretaries or a friend or a relative - a spouse? - where they were going and what they were doing on the night they were murdered? Not one of them? Not one single trace was left for the police to follow? Absolutely impossible.
Continuity mistake: In one of the scenes after the grad students had poisoned their guest, Mark is squatting behind the table such that only his face is visible while he is talking to the others. Watch the wine glass beside him because the level of wine in it keeps changing between shots.