Credit Card rewards. Only a small percent of diligent cardholders will benefit on net from rewards points, cash back, sign up offers, 0% interest periods, etc.

These rewards are paid for by less diligent customers who don't pay off their account each period and pay exorbitant interest rates. People also tend to increase their spending more than their reward rate. Many cards make you jump thru hoops to claim the reward.

Most of the time, these rewards don't even cover third party transaction fees (let alone sales tax). Even if you are diligent, the CC company is still selling data about your spending habits unless you call them and explicitly ask to invoke your right (as US citizen) to opt out of third party data sharing.