Continuity mistake: Peter says "Oh crap" as soon as he realises Ernie the Giant Chicken is looking at him in the window. When the camera pans over to Ernie, he is standing an inch away from the glass. It then cuts to a wide angle where Ernie jumps through the window, and he has instantly moved back a few inches.
Continuity mistake: In the scene with the multiple versions of Stewie and Brian come back to 1999, one Peter is with one of them. When Stewie asks everyone to vote between 9/11, Peter is standing in the foreground, but disappears when one of the Stewies says Brian had his throat slit.
Continuity mistake: When Stewie is playing golf at the top of the stairs before attacking Brian, the layout of the upstairs floor changes quite radically between shots. First Stewie is playing golf with the stairway behind him. Then when Brian puts on the disguise and walks past, there's suddenly a wall behind Stewie. Brian walks off to the left and Stewie begins to chase him that way, but in the next shot Stewie is chasing Brian off to the right. In that same shot you can see that Stewie is running away from the stairs, but in the next shot, without changing direction, he pushes Brian down the stairs, which mysteriously have appeared in front of him.
Continuity mistake: During the scene where the Griffins have Tom Brady over for dinner, when we first see the establishing shot of everyone at the table, Chris is sitting with an empty chair between him and Tom. However, when he asks if he can call him Tom along with Lois and Meg, he is sitting right next to Tom, with no empty chair between the two.