Continuity mistake: When everyone is in the dining room at the dinner table, the order of where everyone sits changes between shots. In all moments where it shows everyone sitting, the order is Priscilla, Derek, Jillian, Mort, Muriel, and Carl. But in one shot, the order is Priscilla, Derek, Jillian, Muriel, Mort, Carl.
Continuity mistake: When Derek Wilcox is killed and on the ground, he is on his front with his head turned. He got hit above his right eye and blood is coming from underneath him, but after everyone else arrives outside, he is on his back with the wound above his left eye and blood is all over his face.
Continuity mistake: When Brian and Stewie arrive in Leonardo's atelier we know that they are in Florence for two reasons: because of history and because we see Palazzo Vecchio (which is in Piazza della Signoria, in Florence) outside the window. Then when Stewie and Bertram are fighting with Leonardo's flying machine the city of Florence is wrongly represented as if it was Venice, with the see, the lagoon and the canals.
Continuity mistake: When Franz Gutentag falls off the step towards the end of the episode, Herbert is on the porch of the house and reaches out to him. When we see the shot of Gutentag falling in slow motion and the birds fly away, there is no sign of Herbert on the porch.
Continuity mistake: When Peter is eating dinner with Lois and her parents, on his plate there is mashed potatoes and meat loaf. Peter shovels potatoes in his mouth then spits them out all over the table. When his plate is shown again, the meat loaf has disappeared from the plate even though he didn't touch it.
Continuity mistake: When Carter destroys the bench after being the first contestant out of wining the old dirt bike, in the digger he backs off screen and you hear a crash and a car alarm, but when the shot is widened you see two cars where he backed into and both are undamaged.
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.
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.