Humans are easily confused though. I've been 100% sure I've run the tests and haven't many times. It's quite easy to do: make a change, run the tests, fix it, run again, everything works. Make some trivial refactor that "shouldn't change anything". Your brain says "it's fine, I ran the tests, remember?" and you push. But you didn't run the tests.

"First you must not fool yourself, and you are the easiest to fool"