We all write bugs, we're human and it's hard to think of everything all of the time.
My point was, when we make a thinking error that causes us to write a bug, that same thinking error means we also don't write that test that could find it. If you had thought of writing a test for the disabled thing when you wrote the code, you also would have written it correctly because you would have had that case in mind then.
Now you have a test, but because someone reported the bug and you fixed it. Now it's almost certainly not useful to have anymore.
I believe in automated tests, but for tricky logic mostly.