> ...Especially if there is a very common refactoring bug and you get a pepperoni pizza instead of a pizza with one of the most natural toppings like pineapple.

Exactly! The DRY'ed example in the first section should read rather:

  def make_hawaiian_pizza():
    make_pizza(["ham","pineapple"])

This demonstrates the omnipresent dangers of untested copy-paste.

"Copy-paste, copy-paste, Will Robinson!!"