When Stewie is sitting on Lois's lap watching TV and Peter comes in, at first his book his blue then it is red.
When Chris shows Peter his drawing in the forest, in the shot before, there was no drawing on either side.
When Brian is watching TV at first the remote is on the right hand side and there is an olive in his glass, but in the next shot the olive is gone and the remote in on the left hand side.
After Lois loses the car the Griffins are all sitting in the hotel room. In the first shot of the hotel room the border at the top of the wall is blue, red and orange. But in every shot after that the border is blue, white and red.
When The Griffins leave to go to New York in their car, Meg and Chris swap places.
When Peter is having his spiritual vision, he asks the Fonz whether he ever got a venereal disease, to which Fonzie replies "herpes twice, and the clap". The problem is that this answer implies that Fonzie's first bout of herpes cleared up, however herpes is incurable making it impossible to contract the disease twice.
The writers can't seem to decide how big of a Happy Days fan Peter is. Early on, he claims he's watching the one where Fonzie says "Heyyyyy." (This is Fonzie's catch phrase and it's spoken frequently, which would imply Peter doesn't know much about the show). But later on, when he has his vision and is speaking to the vision of Fonzie, he knows who the Tuscaderos are, which is something only a die-hard fan of the show would know.