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.
Continuity mistake: After Cleveland cuts open the safe door using Reese Witherspoons's chin, the door falls straight down inside the safe. After Cleveland, Quagmire and Peter step inside, you see that the safe door has mysteriously moved more to the left.
Continuity mistake: Peter gets beat up and thrown in to a black limousine by hired thugs of Mr. Pewterschmidt. While in the car, notice that as Peter starts talking to Mr. Pewterschmidt that there is no cart of Diet Rites next to him. Next shot there is. Then the cart disappears as Mr. Pewterschmidt tells Peter what is on his mind.