That sounds an awful lot like superstition. Superspies from the NSA are not going to
step in on every little case and save the day. People who download a single file by
accident are never going to warrant the huge expense of an attempt, and such
attempts frequently fail if secure deletion software is used.

Besides, people who collect illegal porn apparently collect a lot of it and don't delete
it, so the point is effectively moot. The guys who get caught always have a lot of it,
so imaginary super-CSI wizardry isn't necessary.