Why refactoring is harder than you think, a pictorial representation

Suppose you are working on a legacy code base. It looks like this:

Within it you find a piece of functionality that does a single thing, but is implemented in a hideously complicated way. Like so.

You look at that and think: "That's awful. I know how to do it better. Faster. Easier. More maintainable." Then you set out to do just that. And you succeed in producing this thing.

Wow. Look at that slick and smooth niceness. Time to excise the bad implementation from your code base.

The new one is put in:

And now you are don ... oh, my!