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: The age difference between Chris, Meg and Stewie keeps changing throughout the series. At the beginning of the series, Chris is 13, Meg is 15 and Stewie is 1 (he is after S1 E3 Chitty Chitty Death Bang). In series 4, Chris then goes to high school so he must be 14 by that time. It isn't referenced that Meg has aged between series 1 and series 5 until S5 E10 Peter's Two Dads where she turns 17, so she must have turned 16 during the past five seasons. Despite Meg having aged two years throughout the series, Chris remains having only aged one year since S4 (Age 14 referenced in S9 E13 Trading Places). Then in S10 E10 Meg & Quagmire, Meg then turns 18, and still Chris hasn't aged. Also, Stewie remains one year old after all of Chris and Meg's birthdays. If it were consistent, Meg would be now 18, Chris would now be 16 (not 14) and Stewie would now be 4 (not 1).
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Continuity mistake: Toward the end of this episode, Stewie is at home in the living room talking to Rupert about how depressed he is about being alone. All of a sudden he hears his family come back home, so he gets up from the couch and runs to the window leaving Rupert on the couch. As soon as Stewie moves his head away from the window take notice that Rupert is no longer on the couch.
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: In the episode "FOX-y Lady" (7-10) it was established that Quagmire was 61 years old. In this episode, Quagmire stated to Meg he was bullied in 1986 and a flashback shows that he was a teenager. So how could Quagmire have been bullied in 1986 as a teenager if he should have been in his 30's at this time?
Character mistake: After Stewie and Brian go to tell the past versions of themselves not to tell 1999 Brian about 9/11, Stewie then finds out that Brian then told 1999 Brian to write the Harry Potter novels. This would have been impossible because the first Harry Potter novel was written in 1997, 2 years before the time period Stewie and Brian arrive in.
Factual error: In the episode, the Titanic is shown to have sunk the same day it left England. The actual Titanic left England on April 10 and didn't sink until the night of April 14, almost 4 days later. Plus they show it leaving from London, not Southampton as it did.