Continuity mistake: At the end when everyone is packing their bags and leaving, Jillian is sitting on a bench upset and Bonnie's hand is on her shoulder because Derek was killed. Her neck collar is missing, and her shirt strap is on the right side instead of the left side.
Continuity mistake: When Mayor West panics, Peter shakes him and he falls against a shelf. The plate on the end of the shelf falls off and smashes on the floor. But when Brian points out the gun is there, it is now the middle plate that fell off and the plate on the end of the shelf is intact. (00:15:25)
Continuity mistake: There are several differences during the flashbacks and what actually happened. Such as when Muriel is killed Diane appears with everyone else, but in the flashback she appeared moments later. Second, when everyone appears in front of Derek's dead body there are two differences, Tom doesn't say "Dear God!" in the flashback and Diane appears 1 second later in the flashback. Third, Stephanie doesn't say "Did the cork hit me?" when she is shot in the flashback. Fourth, Priscilla isn't wearing a scarf in the flashback even though when it actually happened she was wearing a scarf. Fifth, when Derek says he is going up to the roof he is standing next to Peter and Lois, in the flashback he standing next to Lois and Jillian. Sixth, when Derek leaves the dining room to go to the balcony, no-one is standing next to the door, but in the flashback Diane and Doctor Hartman are standing right next too the door. Seven, when Muriel is killed she is in the middle of a hallway. But in the flashback, she is killed outside Tom Tucker's room.
Other mistake: When everyone is leaving the mansion after Stephanie was killed, 3 mistakes happen at the same time: Dr. Hartman came alone, but there is a person sitting in the front seat next to him. Derek is in the car he and Jillian came in when they got here, but he is alone. And Carl was getting into Herbert's Ice Cream Truck, which was in front of his own truck.
Other mistake: As Joe is driving up to the house, just before coming to a complete stop, you can see that his window is completely closed. After stopping, Peter says "Joe?" Joe says "Peter?" Joe's window is now fully open. Trouble is, Joe seems to be completely surprised by Peter standing there which implies he never saw Peter even after stopping. His window has mysteriously opened immediately. (00:04:05)
Continuity mistake: When they notice Muriel has disappeared, everyone runs back up the secret passage. As they run out the room, the order of the guests are: Peter, Diane, Chris, Lois, Carl, Doctor Hartman, Meg, Jillian, Brian, Quagmire, Mort, Derek, Seamus and Mayor West is way at the back. Yet as they run through the passage the order is: Peter, Lois, Tom, Quagmire, Diane, Chris, etc. But as they come out of the passage, the order is: Peter, Lois, Tom, Diane, Chris, Bonnie, Meg, Quagmire, Carl, Mayor West, Doctor Hartman, Mort, Seamus and Jillian is way at the back and not Mayor West since he was last seen exiting the secret room. Also Derek nor Consuela were seen in the room just as they ran out, yet when they magically appear when they are doing the groups. (00:21:50)
Continuity mistake: When James returns to the lounge to find everyone there, they all accuse him of killing Stephanie, so they walk over to the dining room with James at the front followed by Tom and Priscilla behind him and Joe and Doctor Hartman behind them. But when they enter the dining room, Tom and Priscilla are no where to be seen and it is now Joe who is behind him. Also in that same scene, several other characters change position. The people who are in the room are Peter, Chris, Brian, Quagmire, Joe, Bonnie, Diane, Doctor Hartman and Seamus and the rest of the guests aren't seen. So when they enter, Diane and Chris are at the back followed by Seamus, Quagmire and Bonnie in front of them, followed by Doctor Hartman, Peter and Joe at the front. There is then a shot of the chair completely empty. Yet in the next shot, Brian has suddenly appeared next to Joe out of nowhere. Then in the following shot, Diane is still at the back but now more closer to Bonnie and both Bonnie and Quagmire have changed positions and Seamus is now at the back and Chris is suddenly next to Quagmire at the edge of the right hand side of the screen and Peter is now behind Joe. Then the shot cuts to James and the shot cuts back to guests. But now Peter has appeared next to Doctor Hartman again and now in front of Joe and not behind him. Also Diane is now no longer at the back but in the middle towards the left of screen and no where Bonnie and Seamus is now partially in front Bonnie. Then when the shot cuts to James saying he is a member of MENSA and a huge hit with the ladies, there is no sign of Brian standing next to Joe. Then in the shot before the lights go out, Diane is now at least ten inches behind Bonnie and Lois has appeared behind Seamus out of nowhere. Then by the time the lights go back on again, Lois is now in front of Joe and all the other guests have appeared in the dining room. Then when Seamus says "Look", there's no one in front of him even though loads of other people were there in front of him before the lights went out. Then the shot zooms out and we can see in the shot are Doctor Hartman, Priscilla, Joe, Brian, Carl, Mort and Tom. Then when Joe says "Nobody touch the knife", it's now Seamus, Bonnie, Quagmire, Lois and Derek in the shot. Then when we see Peter and Chris in the shot, the shot zooms were Joe appears we can see Quagmire, Lois and Bonnie behind Peter even though they were way in front of them a couple of seconds ago. No way could they have all moved that fast, not with all the other people in the way. (00:13:40)
Continuity mistake: When Diane Simmons tells Lois that she is the murderer, Quagmire has already left the manor with Stephanie's body in the boot. When Diane is walking Lois outside, Quagmire is seen again even though he just left. This has been cut from the DVD. (00:45:40 - 00:46:50)