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.
Continuity mistake: When Carl mentions Labyrinth to Tom Tucker and Diane Simmons, Meg can be seen at the other side of the table sitting in front of Joe. But when we next see Meg a few moments later, Doctor Hartman is now sitting where Joe was.
Other mistake: Everything goes backwards in time and goes that far back that Meg (who is voiced by Mila Kunis) goes back to being voiced by Lacey Chabert (who voiced Meg in season 1). However, when Lacey Chabert does her line to Stewie being in the Griffin house for the first time, it goes back to Mila Kunis when Stewie is "reborn". However, if everything was going backwards and Brian fixed everything to make it goes forwards again, then it still should have been Lacey Chabert voicing Meg, not Mila Kunis.
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.
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Audio problem: Just after Peter graduates from the Police Academy, he goes home to Lois and draws his gun in the kitchen, which gets Lois very excited. Every time Peter fires his revolver, you'll hear a soft, but distinctive clink briefly after each shot. This clink comes from shell casings hitting the floor, which revolvers make impossible. Revolvers are designed to eject expended shell casings only while its user is reloading, not after each shot is fired. Only semiautomatic firearms immediately eject expended cartridges immediately after having been fired. Unless I'm mistaken, the entire gunshot sound originally came from a 9mm semiautomatic pistol.
Continuity mistake: As Joe is waiting on Peter, Quagmire and Cleveland at the bar, take notice that he is holding a tray with two bottles of beer on it. One bottle is sitting behind the other in the close up shot of Joe. However when the shot pans out, you see that the bottles are side by side.
Continuity mistake: Right after Lois is telling Peter that he should go to Joe's house, you see a plate and a coffee mug. But right after the angel spirit holds the gun up to Peter's head, and he is walking away from the table, the plate and mug are gone.