We have all made this mistake at one time or another in our web-developer careers: uploading the wrong files to a production webserver.
“Ey, it happens”, you could say. A lot of the time the aware software engineer picks up the obvious error right away, and fixes it before anyone notices.
Yet on many other occasions they sneak by unnoticed until they break down the application on the most unfortunate of times. Like that one make-or-break presentation.