One time I offered to develop film for a friend of mine. I did all of the steps the way I usually develop film, but when I took a look at it after removing it from the tank, it was completely blank! I finally concluded that my friend had not put the roll of film in the camera correctly and it was never exposed.
In the guide from Ilford that you linked, it was the example of "EDGE SIGNING IS VISIBLE BUT NO IMAGES SHOW ON FILM"
At least I can say it was her error and not mine