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.
Continuity mistake: The group squabble at the table over how many they have killed, 9 or 10, with the latest victim still present with a knife in her back. Pete has the last word by taking off his jacket and shirt to reveal the upper part of his arm cast which has ten hash marks on it. He proclaims "It's ten." In that the woman just killed at the table made the tenth victim, how did he already have that tenth hash mark on his cast?
Other mistake: At the end of the play, Arbuthnot picks up a newspaper in the dining room and reads an article with a headline about disappearing local people (referring to those the graduates have killed). However, the actual body of text in the article is talking about politicians voting to halt construction of a new Hadron Collider.