Do people really reason like this? I mean, programming is absolutely clear-cut. It's the most clear-cut aspect of life, in some ways. It's purely a logic problem. If you see something that can't possibly be interacting with your problem, then spending additional mental cycles on it is always a waste of your time for solving that problem.

Now, it may be a good idea to fix that new problem. That's perfectly true. But if it can't possibly affect your current problem, then fixing the new problem won't do a darn thing to help you fix your current problem. That sounds like tautology, but your comment is saying the opposite.

You can prove to yourself that a new problem can't possibly be interacting with your current problem. If the problems exist in two separate modules, you look at the connections between the modules and see whether any data can possibly flow from one to the other. If there is no state that flows between them, then the problems are necessarily independent.

I think what you're saying is that "most codebases suck, because they have a lot of interdependencies and are hard to analyze." That's probably true. But resorting to voodoo thinking isn't going to help.