Serendipity is one of the 16 conversion tactics. It has two sub-categories: Luck and Clairvoyance.
Luck creates the sense that the user is lucky to have come across something on the page.
Clairvoyance creates the sense that the seller predicted something about the user.
This email from TicketMaster is a great example of Luck within Serendipity:
Why is this such a great idea? It may shock you to hear, but only .04% of March Madness brackets stay perfect after the first day of the tournament. TicketMaster knows that a busted bracket might give you more free time (since you probably won’t be winning that $20 work pool). They offer events that are going on during the tournament, which makes the user feel Lucky because they now have something to do.