Ssiscool

Corrected entry: At the beginning of the movie, when the are showing "The Itchy and Scratchy Movie", Itchy launches the nuclear missiles which are initially spread apart but when they land in Scratchy's mouth they are all in one clump.

Correction: You can't seriously expect perfect continuity watching a cartoon within a cartoon.

But then itchy and scratchy had to be made as part of the Simpson's movie so it's still part of the film making it a mistake.

Ssiscool

Wrong. This is not a mistake. It was fully intentional and done for comical purposes.

Correction: Missiles can also change trajectory.

Phaneron

It was done as deliberate joke, so it isn't a mistake.