An error (or slip) that occurs whilst typing in the URL box and one unconsciously begins typing in the URL of one's favorite porn site that leads one from a simple wikipedia search to a 3 hour jerkoff session. Like the Freudian slip, it is usually caused by an interference of some unconscious wish, conflict, or train of thought.
Damn you pornian slip!! I just wanted to simply check my email and now I have 5 tabs of pornhub.com opened!!
It is studied under the division of psych called pornanalysis and pornian slip is often referred to as parapsexis.