> Don’t be tempted to skip the manual test because you think the automated test has you covered already! Almost every time I’ve done this myself I’ve quickly regretted it.

How does this work? When expectations about the program's state vs its observable behavior diverge?

Just one very simple example: you add a new frontend feature where clicking a button opens a modal. You include an automated test that selects the button and clicks it using document.querySelector("#mybutton").click() - and the test passes. Then when you test it in a browser yourself you find that the button is impossible to click because it's invisibly positioned behind some other element.

A manual test is often an end-to-end test which are notoriously difficult to automate.