What happens when you click on a link/video, thinking it's something of interest (like porn, free online movie/show, or something your friend recommended), but it leads you to the music video for Rick Astley's "Never Gonna Give You Up".
This trick is pretty common because the video or link looks promising with an according title and thumbnail.
(NOTE: Very common on Youtube)
Man 1: "Hey dude, I'm sending you this link to this killer porn site! Best ever!"