Chapter 11. Accommodating Change
Some problems become most apparent when you try to change your code. (Most of the other smells we've discussed can be detected by looking at the code statically.)
Ideally, one changed decision affects only one place in the code. When it doesn't work out that way, it's a sign of duplication in the code.
Addressing these smells often has a side benefit: Many times it makes the code easier to test.