As Lois is being shot, you can see that she has several bullet holes in her back, but as she sinks into the ocean, they have completely vanished.
When Stewie shoots Lois, the number of bullet holes changes from shot to shot.
When Stewie takes the drawings and the gun out of his safe, he puts them in a sack that is later shown tied at the top. But when Joe retrieves the items, he simply reaches into the garbage can and pulls them out.
In the cutaway with the Beatles, John Lennon is shown playing a bass guitar. He was the band's rhythm guitarist and played a standard 6-string guitar.
Peter warns Brian to stay out of the cat box. Even though the Griffins only had a cat when Brian moved out and never when he was living with them, Peter (as dumb as he is) said it in a joking tone.
When Stew kills Lois he used a non silenced gun someone should have heard it.