A problem I’ve found is that when you’re adding functionality or refactoring it often leaves unused methods or types behind, at least with multiple devs working on the same codebase.

This unused code gets further modified as time goes on: new functionality is wired in, or it gets further refactored. Usually it’ll still have tests that cover it. It gives the impression of being live code, but it’s not: it’s zombified.

So you get situations where it gets wired up to something and then that something doesn’t work and you wonder why and so you start digging about and you discover it’s because it has been wired into a path that is never executed.

The fog of relatively recent changes sometimes makes it hard to figure out if the code should be unused or if someone just forgot to hook it in as part of a bigger piece of work. Then you find nobody else is really sure either.

So that extra complexity comes at a cost. It can slow you down or trip you up; catch you by surprise.