The part that gets me is when it passes lint, passes tests, and the logic is technically correct, but it quietly changed how something gets called. Rename a parameter. Wrap a return value in a Promise that wasn't there before. Add some intermediate type nobody asked for. None of that shows up as a failure anywhere. You only notice three days later when some other piece of code that depended on the old shape breaks in a way that has nothing to do with the original change.