There's understanding and there's understanding.

Have you never "fixed a bug", only to realize that you just papered over a single symptom, while the underlying bug is still intact?

People you're disagreeing with (I think!), would say that during your first attempt, you didn't _really_ understand the part you're modifying.

It is _very easy_ to do this in large codebases, and even more so when working on anything touching UI.