When during class the physics teacher starts talking about his adventures where he saves his wife from drowning. Then he stands up and slaps every student with his massive shlong. He then goes on to give out free backshots and the principal also gets some.
The equivalent of a Rick Roll on YouTube, but with a Justin Bieber video, instead of Rick Astley.
Can also happen on torrent when you think you are downloading a James Bond movie called "Never Say Never" and then find out it's a Justin Bieber movie.