The Cancel Button
The cancel/kill button is one of those most essential elements in any piece of technology, even if it’s never used.
When we input, we make mistakes. We should be able to change the output before our request has been processed (or gets stuck in processing). Why don’t elevators have a cancel button when you hit the wrong floor? Why don’t all printers have an off switch as opposed to a button which has to interact with the firmware, which could be non-responsive anyway.